Student acted oddly (5)
I believe the answer is:
cadet
'student' is the definition.
(I have seen 'Military student ' mean 'cadet' so perhaps 'student' could also mean 'cadet')
'acted oddly' is the wordplay.
'oddly' is an anagram indicator.
'acted' anagrammed gives 'CADET'.
(Other definitions for cadet that I've seen before include "Younger son's branch of a family" , "Military student" , "Younger member of family" , "Military trainee officer" , "Officer-trainee" .)