Aye's opposite (3)
I believe the answer is:
nay
'aye's opposite' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for nay that I've seen before include "Negative answer" , "No (dialect); or rather" , "In point of fact - a parliamentary vote against" , "Refusal" , "telling a nicker from a pony?" .)