Thrash in return game (4)
I believe the answer is:
flog
'thrash' is the definition.
(I've seen this before)
'in return game' is the wordplay.
'in return' says the letters should be written in reverse (return can mean to go backwards).
'game' becomes 'golf' (golf game is a kind of game).
'golf' reversed gives 'FLOG'.
(Other definitions for flog that I've seen before include "Beat with a whip or rod" , "Lash" , "Hit - sell" , "Scourge, whip" , "- - a dead horse" .)