Musical performance involving clarinet, not new (7)
I believe the answer is:
recital
'musical performance' is the definition.
(I've seen this before)
'involving clarinet not new' is the wordplay.
'involving' indicates an anagram.
'not' is a deletion indicator.
'new' becomes 'n' (abbreviation as in New Testament NT).
'clarinet' with 'n' taken out is 'clariet'.
'clariet' is an anagram of 'RECITAL'.
(Other definitions for recital that I've seen before include "Verbal account" , "Soloist's performance" , "gig" , "Article found in performance" , "Reading" .)