Piece of music available in bookshop, usually (4)
I believe the answer is:
opus
'piece of music' is the definition.
(synonyms)
'available in bookshop usually' is the wordplay.
'available in' indicates a hidden word.
'OPUS' can be found hidden inside 'bookshop usually'.
(Other definitions for opus that I've seen before include "Work (of a composer?)" , "Piece of music (with number)" , "Composition - magnum __" , "(Numbered) musical composition" , "Musical composition in the soup" .)