Mark from Flanders leaving France for Britain (7)
I believe the answer is:
blemish
'mark' is the definition.
(I've seen this before)
'from flanders leaving france for britain' is the wordplay.
'from flanders' becomes 'Flemish' (I've seen this before).
'leaving' is a deletion indicator.
'france' becomes 'f'.
'for' is a charade indicator (letters next to each other) (I've seen this in other clues).
'britain' becomes 'B' (common abbreviation as in GB).
'flemish' with 'f' taken out is 'lemish'.
'lemish' put after 'b' is 'BLEMISH'.
(Other definitions for blemish that I've seen before include "deformity" , "Stain - defect" , "Moral defect or fault" , "Flaw; moral defect" , "bad mark?" .)