Very French advance must be a sin (8)
I believe the answer is:
trespass
'a sin' is the definition.
(I've seen this before)
'very french advance' is the wordplay.
'very french' becomes 'tres' ('very' translated into French).
'advance' becomes 'pass' (passing is a kind of advancing).
'tres'+'pass'='TRESPASS'
'must be' is the link.
(Other definitions for trespass that I've seen before include "Enter someone else's property illegally" , "offence" , "Action of intruder" , "Entry without permission" , "Enter another's property without permission" .)