Four players required for this violin part? (6)
I believe the answer is:
bridge
I believe this clue is a double definition.
'four players required' is the first definition.
(Bridge is a four-player card game)
'this violin part?' is the second definition.
(I've seen this before)
'for' is the link.
(Other definitions for bridge that I've seen before include "See 14" , "Card game for four players" , "One on the fiddle" , "Where four hands are needed" , "A walkover" .)