Game's up! Sell! (4)
I believe the answer is:
flog
'sell' is the definition.
(I know that sell can be written as flog)
'game's up' is the wordplay.
'game' becomes 'golf' (golf is an example).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'golf' back-to-front is 'FLOG'.
(Other definitions for flog that I've seen before include "Hit - sell" , "Keep hitting" , "Whip severely" , "- - a dead horse" , "Lash" .)