Trainee acted strangely (5)
I believe the answer is:
cadet
'trainee' is the definition.
(cadet is a kind of trainee)
'acted strangely' is the wordplay.
'strangely' indicates anagramming the letters.
'acted' is an anagram of 'CADET'.
(Other definitions for cadet that I've seen before include "young soldier" , "not the heir" , "Trainee soldier" , "Trainee cop" , "Police or armed forces trainee" .)