Justification of French criminal (7)
I believe the answer is:
defence
'justification' is the definition.
(I know that defence is a type of justification)
'of french criminal' is the wordplay.
'of french' becomes 'de' ('of' in French).
'criminal' becomes 'fence' (someone who handles stolen goods).
'de'+'fence'='DEFENCE'
(Other definitions for defence that I've seen before include "A means of protecting from attack" , "The D in MoD" , "barricade" , "Weaponry, possibly" , "Military protection" .)