Trainee soldier, a rotter, meets an alien (5)
I believe the answer is:
cadet
'trainee soldier' is the definition.
(I've seen this before)
'a rotter meets an alien' is the wordplay.
'a rotter' becomes 'cad' (a cad is a dishonourable person).
'meets' means one lot of letters go next to another.
'an alien' becomes 'ET' (extra terrestrial).
'cad'+'et'='CADET'
(Other definitions for cadet that I've seen before include "not the heir" , "Trainee -- acted (anag)" , "Trainee cop" , "Police force trainee" , "recruit" .)