Helpful first violinist? (15)
I believe the answer is:
instrumentalist
'violinist?' is the definition.
(violinist is a kind of instrumentalist)
'helpful first' is the wordplay.
'helpful' becomes 'instrumental' ('instrumental' can be similar in meaning to 'helpful').
'first' becomes 'ist'.
'instrumental'+'ist'='INSTRUMENTALIST'
(Other definitions for instrumentalist that I've seen before include "one of the band" , "oboe, perhaps" , "musician" .)