Numbered musical piece (4)
I believe the answer is:
opus
'numbered musical piece' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for opus that I've seen before include "Composer's work" , "Work (mus.)" , "Work of 25 perhaps" , "Major work" , "Creative work, eg in writing, art or music" .)