Repeal a French act (4)
I believe the answer is:
undo
'repeal' is the definition.
(thesaurus)
'a french act' is the wordplay.
'a french' becomes 'un' ('a' in French).
'act' becomes 'do' (I've seen this in other clues).
'un'+'do'='UNDO'
(Other definitions for undo that I've seen before include "Ruin or loosen ties" , "Loosen; annul" , "Reverse effect of" , "Destroy - nullify" , "Subvert" .)