Rating so poor for musician (8)
I believe the answer is:
organist
'musician' is the definition.
(organist is a kind of musician)
'rating so poor' is the wordplay.
'poor' indicates anagramming the letters.
'rating'+'so'='ratingso'
'ratingso' is an anagram of 'ORGANIST'.
'for' is the link.
(Other definitions for organist that I've seen before include "Cathedral musician" , "It groans for church musician" , "Accompanist, keyboard player" , "Church instr. player" , "Roasting (anag)" .)