Trainee acted badly (5)
I believe the answer is:
cadet
'trainee' is the definition.
(cadet is a kind of trainee)
'acted badly' is the wordplay.
'badly' indicates anagramming the letters (letters in the wrong order).
'acted' with letters rearranged gives 'CADET'.
(Other definitions for cadet that I've seen before include "Younger son's branch of a family" , "Trainee cop" , "aspiring officer" , "officer of the future" , "novice" .)