Arm going up to win gold (6)
I believe the answer is:
nugget
'gold' is the definition.
(thesaurus)
'arm going up to win' is the wordplay.
'arm' becomes 'gun' (gun is a kind of arm).
'going up' says the letters should be written backwards (in a down clue, the letters are written upwards).
'to win' becomes 'get'.
'gun' in reverse letter order is 'nug'.
'nug'+'get'='NUGGET'
(Other definitions for nugget that I've seen before include "Piece of advice" , "valuable fragment" , "Solid lump of a precious metal" , "Followed on" , "Lump of, eg, gold" .)