In the third sentence of the text, “supreme” is an adjective modifying the noun “perfection”.

Text IV

In the middle of July, Roger was making his first ministerial speech. I did not need reminding, having drafted enough of them, how much speeches mattered — to parliamentary bosses, to any kind of tycoon. Draft after draft: the search for the supreme, the impossible, the more than Flaubertian perfection; the scrutiny for any phrase that said more than it ought to say, so that each speech at the end was bound, by the law of official inexplicitness, to be more porridge-like than when it started out in its first draft. I had always hated writing drafts for other people, and nowadays got out of it. To Hector, to Douglas, it was part of the job, which they took with their usual patience, their usual lack of egotism: when a minister crossed out their sharp, clear English and went in for a literary composition of his own, they gave a wintry smile and let it stand.

C. P. Snow. Corridors of Power. London: Penguin Books, 1972, p. 31.