Musical performance playing clarinet, not new (7)
I believe the answer is:
recital
'musical performance' is the definition.
(thesaurus)
'playing clarinet not new' is the wordplay.
'playing' is an anagram indicator.
'not' indicates named letters should be taken away.
'new' becomes 'n'.
'clarinet' with 'n' taken out is 'clariet'.
'clariet' is an anagram of 'RECITAL'.
(Other definitions for recital that I've seen before include "statement of facts" , "Reading" , "(Usually solo) concert" , "Performance of music by soloist" , "Article found in performance" .)