Acted as a trainee (5)
I believe the answer is:
cadet
'a trainee' is the definition.
(cadet is a kind of trainee)
'acted' is the wordplay.
'acted' can be anagrammed to 'CADET'.
However, I am not sure how this is indicated.
'as' acts as a link.
Can you help me to learn more?
(Other definitions for cadet that I've seen before include "novice" , "One in training" , "A trainee army officer" , "Armed services trainee" , "Younger member of family" .)