Plea of French criminal (7)
I believe the answer is:
defence
'plea' is the definition.
(thesaurus)
'of french criminal' is the wordplay.
'of french' becomes 'de'.
'criminal' becomes 'fence' (I've seen this before).
'de'+'fence'='DEFENCE'
(Other definitions for defence that I've seen before include "Armed forces' role" , "One ministry" , "This department deals with national protection" , "barricade" , "Military's purpose" .)