Opposite of "oui” (3)
I believe the answer is:
non
'opposite of oui' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for non that I've seen before include "Negative French word" , "Prefix meaning negative" , "Prefixed negation of a word" , "Prefix meaning not or none" , "Combining form meaning not" .)