Work found in bookshop, usually (4)
I believe the answer is:
opus
'work' is the definition.
(I've seen this before)
'in bookshop usually' is the wordplay.
'in' indicates a hidden word.
'OPUS' can be found hidden inside 'bookshop usually'.
'found' is the link.
(Other definitions for opus that I've seen before include "Piece of music (with number)" , "Artistic work" , "Creative work, eg in writing, art or music" , "Composer's work" , "Work (mus.)" .)