Stop Frenchmen entering company area (5)
I believe the answer is:
comma
'stop' is the definition.
(I've seen this before)
'frenchmen entering company area' is the wordplay.
'frenchmen' becomes 'MM' (two M's - M is short for Monsieur).
'entering' is an insertion indicator.
'company' becomes 'co' (abbreviation).
'area' becomes 'a' (maths abbreviation).
'co'+'a'='coa'
'mm' inserted within 'coa' is 'COMMA'.
(Other definitions for comma that I've seen before include "Common punctuation park" , "Short break" , "Puncuation mark" , "shortest interval" , "Kind of butterfly" .)