Nay's opposite (3)
I believe the answer is:
'nay's opposite' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for yea that I've seen before include "Verily" , "Archaic affirmative answer" , "Certainly (archaic)" , "indeed" , "Aye; aye (anag.)" .)