New cadet at university behaved badly (5,2)
I believe the answer is:
acted up
'behaved badly' is the definition.
(acting up is a kind of behaving badly)
'new cadet at university' is the wordplay.
'new' indicates anagramming the letters.
'at university' becomes 'up' ('up' can mean studying at university).
'cadet' anagrammed gives 'acted'.
'acted'+'up'='ACTED UP'
(Other definitions for acted up that I've seen before include "Behaved badly" , "carried on" .)