The game is up for a valuable violin (5)
I believe the answer is:
strad
'a valuable violin' is the definition.
(I've seen this in another clue)
'the game is up' is the wordplay.
'the game' becomes 'darts' (darts is a kind of game).
'is up' says the letters should be written backwards (in down clue: letters go upwards).
'darts' in reverse letter order is 'STRAD'.
'for' is the link.
(Other definitions for strad that I've seen before include "that Grappelli could have played" , "Noted violin" , "Valuable old violin" , "Old, very expensive violin (abbr)" , "fiddle exposed" .)