Student acted badly (5)
I believe the answer is:
cadet
'student' is the definition.
(I have seen 'Military student ' mean 'cadet' so perhaps 'student' could also mean 'cadet')
'acted badly' is the wordplay.
'badly' indicates anagramming the letters.
'acted' with letters rearranged gives 'CADET'.
(Other definitions for cadet that I've seen before include "Police force trainee" , "officer of the future" , "Officer in training" , "Armed services trainee" , "A trainee army officer" .)