A French and Italian fighting force (4)
I believe the answer is:
unit
'fighting force' is the definition.
(thesaurus)
'a french and italian' is the wordplay.
'a french' becomes 'un' ('a' in French**).
'and' is a charade indicator (letters next to each other).
'italian' becomes 'it'.
'un'+'it'='UNIT'
(Other definitions for unit that I've seen before include "Individual thing as part of a whole" , "Individual thing, single and complete" , "Individual component" , "A considerable number of soldiers" , "squad" .)