Help required by second-rate string player (7)
I believe the answer is:
'string player' is the definition.
'bassist' can be an answer for 'player' (I've seen this before). I am unsure of the 'string' bit.
'help required by second-rate' is the wordplay.
'help' becomes 'assist' (to assist someone is to help them).
'required by' is a charade indicator (letters next to each other).
'second rate' becomes 'b'.
'assist' after 'b' is 'BASSIST'.
Can you help me to learn more?
(Other definitions for bassist that I've seen before include "jazz player" , "Musician" .)