Tees had the ne plus ultra, ensnaring old fool (5)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

I’D(contraction of “I had” or “Tees had”/”setter had” using the self-referential pronoun) + IT(the ultimate, or, in Latin, “ne plus ultra”, as in “This is it! No more!”) containing ( ensnaring ) O(abbrev. for “old”).