New cadet behaved (5)
I believe the answer is:
acted
'behaved' is the definition.
(acting is a kind of behaving)
'new cadet' is the wordplay.
'new' indicates an anagram.
'cadet' is an anagram of 'ACTED'.
(Other definitions for acted that I've seen before include "appeared on stage" , "exerted influence" , "did something!" , "participated in panto maybe" , "Behaved or performed" .)