Trainee acted oddly (5)
I believe the answer is:
cadet
'trainee' is the definition.
(cadet is a kind of trainee)
'acted oddly' is the wordplay.
'oddly' indicates anagramming the letters (letters in an odd order).
'acted' anagrammed gives 'CADET'.
(Other definitions for cadet that I've seen before include "Police force trainee" , "Apprentice officer" , "A trainee army officer" , "Armed services trainee" , "Younger member of family" .)