Top case involving Frenchmen (6)
I believe the answer is:
summit
'top' is the definition.
('summit' can be a synonym of 'top')
'case involving frenchmen' is the wordplay.
'case' becomes 'suit' ('suit' can be a synonym of 'case').
'involving' is an insertion indicator.
'frenchmen' becomes 'mm'.
'suit' going around 'mm' is 'SUMMIT'.
(Other definitions for summit that I've seen before include "international diplomacy" , "Topmost point" , "Highest point (of hill)" , "Top of a mountain, or meeting between heads of government" , "Apex" .)