Trainee, rotter, extremely efficient (5)
I believe the answer is:
cadet
'trainee' is the definition.
(cadet is a kind of trainee)
'rotter extremely efficient' is the wordplay.
'rotter' becomes 'cad' (a cad is a dishonourable person).
'extremely' means to remove the middle letters (only the extremes of the word are used).
'efficient' with its middle removed is 'et'.
'cad'+'et'='CADET'
(Other definitions for cadet that I've seen before include "Younger member of family" , "Young trainee, eg in the police" , "who needs to be properly trained?" , "not the heir" , "Police force trainee" .)