Church musician often representing him as rector (11)
I believe the answer is:
choirmaster
'church musician often representing him as rector' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'representing him as rector' is the wordplay.
'representing' is an anagram indicator (letters must be re-presented in a new way).
'him'+'as'+'rector'='himasrector'
'himasrector' is an anagram of 'CHOIRMASTER'.
Can you help me to learn more?
(Other definitions for choirmaster that I've seen before include "Singers' leader" , "Director of singers" .)