The French gentleman, person half forgotten, set up to get revenge (8)
I believe the answer is:
reprisal
'revenge' is the definition.
(I know that reprisal is a type of revenge)
'the french gentleman person half forgotten set up' is the wordplay.
'the french' becomes 'la' (one of the French words for 'the').
'gentleman' becomes 'sir' (a man may be addressed as sir).
'half forgotten' indicates taking half.
'set up' shows that the letters should be reversed in order (in a down clue, letters go up).
'person' cut in half is 'per'.
'la'+'sir'+'per'='lasirper'
'lasirper' back-to-front is 'REPRISAL'.
'to get' is the link.
(Other definitions for reprisal that I've seen before include "compensation" , "revenge attack" , "Retaliatory action against an enemy" , "hitting back" , "Act of retaliation or vengeance" .)