Habit of organist’s regularly entertaining pieces (7)
I believe the answer is:
raiment
'habit' is the definition.
(thesaurus)
'organist's regularly entertaining pieces' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'entertaining' is an insertion indicator (inviting in, as for dinner).
'pieces' becomes 'men' (man can mean a chess piece).
The alternating letters of 'organists' are 'rait'.
'rait' going around 'men' is 'RAIMENT'.
'of' acts as a link.
(Other definitions for raiment that I've seen before include "Attire" , "Clobber" , "Am inert (anag) - clothing" , "Poetic word for clothing" , "Clothing (literary)" .)