Game is up for valuable violin (5)
I believe the answer is:
strad
'valuable violin' is the definition.
(I've seen this in another clue)
'game is up' is the wordplay.
'game' becomes 'darts' (darts is a kind of game).
'is up' is a reversal indicator (in down clue: letters go upwards).
'darts' reversed gives 'STRAD'.
'for' is the link.
(Other definitions for strad that I've seen before include "Famous violin" , "Valuable violin (colloq.)" , "fiddle exposed" , "musical instrument" , "Valuable old violin" .)