Friend and Monsieur dined (4)
I believe the answer is:
mate
'friend' is the definition.
(mate is a kind of friend)
'monsieur dined' is the wordplay.
'monsieur' becomes 'm'.
'dined' becomes 'ate'.
'm'+'ate'='MATE'
'and' acts as a link.
(Other definitions for mate that I've seen before include "China" , "Spouse or informal friend" , "Match" , "assistant" , "Better half" .)