One’s composed — there’s nothing the matter? (4)
I believe the answer is:
opus
'one's composed' is the definition.
(term for a musical work)
'nothing the matter?' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'the matter?' becomes 'pus' (pus is a kind of matter**).
'o'+'pus'='OPUS'
'there's' is the link.
(Other definitions for opus that I've seen before include "Composer's work" , "Work or composition, usually musical" , "Composition - magnum __" , "Work (of a composer?)" , "Piece of music (with number)" .)