ויהי בישורון מלך, "He became King in Yeshurun." [continuing the author's approach based on the "plain meaning", Ed.] every Levite would be expected to declare that there was a king in Israel, i.e. none other than the Lord Himself. The concept is that the presence of the Shechinah over the Jewish camp was a manifestation of the presence of the king. The reason the expression Yeshurun has been used here is that it reflects a visual experience, i.e. the experience of the Israelites at Mount Sinai. At that time they had been enabled to have a visual experience of the attribute of כבוד. This was where all the heads of the tribes had assembled together with all the tribes in order to accept God as their King. [Compare Deut. 32,8 where the author explained the word ישורון, as the same as אשורנו in Numbers 24,17 where Bileam uses is to describe his visionary powers. Ed.]
A Midrashic approach (Pessikta Zutrata): The "king" referred to in the line ויהי בישורון מלך, is Moses, seeing that people had heard the entire Torah (legislation) from his mouth. The Torah means that the manner in which all the people including their tribal heads assembled around him, gave Moses the stature of king. This was not the first time that Moses had acted as lawgiver. It had occurred in Deut. 31,9 already when he had given the Torah to the priests, the Levites, carriers of the Holy Ark. We also find Moses ordering all the people to appear in his presence in Deut. 31,28.
Another way of understanding our verse is that the "king" described here is God Himself. The Torah describes that God became king only when all the tribal heads assembled. We find something similar described in Psalms 47,8-10 where David refers to God's position as "king" in connection with the leaders of mankind indicating that they accept Him as such. In the case of the Jewish people: "when the leaders of the people volunteered to rally around Him." We have another similar description of God becoming king in Amos 9,6. [The prophet portrays that God cannot build His chambers in heaven until an assembly of His loyal people have laid the foundation for this on earth. Ed.] The common denominator in all these verses is that God does not impose Himself as "king," but waits until His creatures manifest their desire for Him to be their king.
A kabbalistic approach: the word ויהי בישורון מלך, is an allusion to the emanation מלכות, which is the one emanation which has absorbed input from all the other emanations "above" it. This is the background to the commentary on the verse in Amos 9,6 הבונה בשמים מעלותיו ואגודתו על ארץ יסדה, "Who builds (commences the structure) in the heaven and gathers it all together when it reaches earth;" [my translation, Ed.] in other words, you may perceive of God's "structure" described by the prophet as if it consisted of a palace built on a number of ships, in sections. As long as the ships supporting different parts of this palace are securely tied together the palace is not in any danger of crumbling; the structure which the prophet describes as the universe, God's palace, i.e. heaven, is secure only as long as Israel provides its sound foundation on earth. This can only be when the Israelites themselves form a firm union one with the other, אגודתו. In the absence of the proper אגודה, "union, brotherliness," of the Jewish people on earth, His throne in the celestial spheres may be viewed as somehow defective, insecure. The word מעלתו, though spelled in the singular, i.e. "His celestial region," is to be read as if it had been spelled מעלותיו (pl.) i.e. "His celestial regions." This is an allusion to the ten emanations. The letter י in the way we read the word alludes to the highest [in our diagrams lowest, Ed.]. emanation, the one which includes input from all the others. This indicates that this verse in Amos makes God's throne's completeness conditional on the condition of the Jewish people on earth, especially their brotherliness, their ability to establish a single אגודה, union.
You should remain aware that the word ויהי in our verse teaches that this king is the Lord God. Furthermore, in the word ויהי you find three of the permutations of the holy name of God, the tetragram, when you perceive of that name as capable of 4 permutations. The four permutations would be יהו'ה, יה'ו, י'ה, י, the last three being capable of construction from the word ויהי. The sequence would correspond to the fruit used as part of the 4 species on Sukkot; [We use a progressively smaller number of each of these species, i.e. 3 Hadassim, 2 Aravot, 1 Lulav in the אגודה, i.e. the three species which form part of the bunch. Ed.]
Seeing the word ויהי is emanated from the tetragrammaton, this indicates that the tenth emanation belongs to God. The meaning of the word מלך in our verse may be similar to what David had in mind when he said in psalm 145,1 ארוממך אלוה-י המלך, "let me extol You, my God and king." It may also correspond to the way the word is used in Numbers 23,21 ה' אלוה-יו עמו ותרועת מלך בו, "the Lord their God is with them and their king's acclaim in their midst." Solomon may have used the word in a similar sense when he said in Kohelet 5,8 מלך לשדה נעבד, "even a king is indebted to the soil." The expression מלך is used in that sense (also by Solomon) in Song of Songs 7,6 when he said מלך אסור ברהטים, "a king is held captive by (nazarite) tresses." The common denominator of the verses we have mentioned is that even a king is subject to restrictions. When God is described as king, this does not necessarily mean His total independence, but His dependence on His subjects in a certain way. [All of this is part of what our sages call His ענוותנותו, "His self-imposed humility." Ed.] Similarly, the verse ה' צבאות הוא מלך הכבוד סלה, "the Lord of Hosts, He is the king of the attribute כבוד." [If I understand the author correctly, the latter expression, i.e. king of an attribute, even that of כבוד, is less encompassing than the title represented by the tetragrammaton. All of this shows that God imposes restrictions upon Himself. The reason that the psalmist added the word סלה was so that we should not err and think that when God manifests Himself as an attribute, כנוי, that this represents His essence. Ed.]