Help musician losing lead (6)
I believe the answer is:
assist
'help' is the definition.
(to assist someone is to help them)
'musician losing lead' is the wordplay.
'musician' becomes 'bassist' (bass guitarist).
'losing lead' means to remove the first letter.
'bassist' with its initial letter removed is 'ASSIST'.
(Other definitions for assist that I've seen before include "Be useful to" , "Work as a subordinate" , "Pitch in" , "Serve" , "Give a hand to" .)