Beatle originally featured on 'Help!' -- McCartney? (7)
I believe the answer is:
bassist
'mccartney?' is the definition.
(Paul McCartney is a bassist)
'beatle originally featured on help' is the wordplay.
'originally' indicates taking the first letters.
'featured on' is a charade indicator (letters next to each other) (in a down clue, some letters go on top of others).
'help' becomes 'assist' (to assist someone is to help them).
The first letter of 'beatle' is 'b'.
'b'+'assist'='BASSIST'
(Other definitions for bassist that I've seen before include "jazz player" , "Musician" .)