Game's up seeing fiddle? (5)
I believe the answer is:
strad
'fiddle?' is the definition.
(thesaurus)
'game's up' is the wordplay.
'game' becomes 'darts' (darts is a kind of game).
'up' is a reversal indicator (in down clue: letters go upwards).
'darts' in reverse letter order is 'STRAD'.
'seeing' is the link.
(Other definitions for strad that I've seen before include "fiddle exposed" , "Top-quality violin (abbr.)" , "Noted violin" , "musical instrument" , "Famous violin" .)