Musician begins overture with help (7)
I believe the answer is:
bassist
'musician' is the definition.
(bass guitarist)
'begins overture with help' is the wordplay.
'overture' indicates taking the first letters (I've seen 'overtures' mean this).
'with help' becomes 'assist' (I've seen this before).
The first letter of 'begins' is 'b'.
'b'+'assist'='BASSIST'
(Other definitions for bassist that I've seen before include "Musician" , "jazz player" .)