Performance playing clarinet's not new (7)
I believe the answer is:
recital
'performance' is the definition.
(recital is a kind of performance)
'playing clarinet's not new' is the wordplay.
'playing' indicates an anagram.
'not' indicates named letters should be taken away.
'new' becomes 'n'.
'clarinet' with 'n' taken away is 'clariet'.
'clariet' anagrammed gives 'RECITAL'.
(Other definitions for recital that I've seen before include "Article found in performance" , "Musician's performance" , "Public performance of music" , "gig" , "Soloist's performance" .)