Rowdy behaviour is unusual in session (9)
I believe the answer is:
noisiness
'rowdy behaviour' is the definition.
(noisiness can mean being rowdy)
'unusual in session' is the wordplay.
'unusual' is an anagram indicator.
'in'+'session'='insession'
'insession' with letters rearranged gives 'NOISINESS'.
'is' acts as a link.
(Other definitions for noisiness that I've seen before include "Clamour" , "conspicuous quality" , "Decibel reading" , "The racket" .)