Military trainee acted inappropriately (5)
I believe the answer is:
cadet
'military trainee' is the definition.
(I've seen this in another clue)
'acted inappropriately' is the wordplay.
'inappropriately' indicates anagramming the letters (I've seen this in other clues).
'acted' is an anagram of 'CADET'.
(Other definitions for cadet that I've seen before include "Potential officer" , "Officer-trainee" , "Armed services trainee" , "Military trainee officer" , "not the heir" .)