Man pledged backing from French Resistance (8)
I believe the answer is:
defiance
'resistance' is the definition.
(defy can mean to oppose or resist)
'man pledged backing from french' is the wordplay.
'man pledged' becomes 'fiance' (a man who is pledged in marriage).
'backing' means one lot of letters go next to another.
'from french' becomes 'de' ('from' in French).
'fiance' put after 'de' is 'DEFIANCE'.
(Other definitions for defiance that I've seen before include "open disobedience" , "Open resistance" , "bold opposition" , "Resistance to authority" , "Disregard for authority" .)