Young trainee acted badly (5)
I believe the answer is:
cadet
'young trainee' is the definition.
(eg an army cadet)
'acted badly' is the wordplay.
'badly' is an anagram indicator (letters in the wrong order).
'acted' with letters rearranged gives 'CADET'.
(Other definitions for cadet that I've seen before include "officer of the future" , "Trainee cop" , "One in training" , "Young trainee, eg in the police" , "A trainee army officer" .)