Acted strangely for a Service trainee (5)
I believe the answer is:
cadet
'a service trainee' is the definition.
(I've seen this in another clue)
'acted strangely' is the wordplay.
'strangely' is an anagram indicator.
'acted' anagrammed gives 'CADET'.
'for' is the link.
(Other definitions for cadet that I've seen before include "who needs to be properly trained?" , "Young trainee, eg in the police" , "A trainee army officer" , "not the heir" , "Younger son's branch of a family" .)