Monsieur's agreement (3)
I believe the answer is:
oui
'monsieur's agreement' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for oui that I've seen before include "Yes, mademoiselle" , "Certainly, monsieur (3)" , "Yes, madame" , "Yes, monsieur" .)