Textbook used by the organist (6)
I believe the answer is:
manual
'textbook' is the definition.
(thesaurus)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for manual that I've seen before include "Work-book" , ""How-to" book" , "Instruction book prepared by hand?" , "Relating to the hands" , "Operated by hand" .)