Aid musician starting late (6)
I believe the answer is:
assist
'aid' is the definition.
(assisting is a kind of aiding)
'musician starting late' is the wordplay.
'musician' becomes 'bassist' (I've seen this before).
'starting late' means to remove the first letter (the word starts later than the beginning).
'bassist' with its initial letter taken off is 'ASSIST'.
(Other definitions for assist that I've seen before include "Serve" , "Give aid" , "Aid, abet" , "Expedite" , "provide backing" .)