Trainee acted wildly (5)
I believe the answer is:
cadet
'trainee' is the definition.
(cadet is a kind of trainee)
'acted wildly' is the wordplay.
'wildly' 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" , "Young trainee, eg in the police" , "Tomboy" , "A trainee army officer" , "Trainee -- acted (anag)" .)