Frenchmen engaged in case conference (6)
I believe the answer is:
summit
'conference' is the definition.
(as in a political summit)
'frenchmen engaged in case' is the wordplay.
'frenchmen' becomes 'MM' (two M's - M is short for Monsieur).
'engaged in' indicates putting letters inside.
'case' becomes 'suit' (synonyms).
'mm' inserted inside 'suit' is 'SUMMIT'.
(Other definitions for summit that I've seen before include "Talks" , "High point" , "conference?" , "Highest point of a mountain" , "Highest point (of hill)" .)