Rotter: extremely efficient trainee (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' (I've seen this before).
'extremely' means to remove the middle letters.
'efficient' with its middle taken out is 'et'.
'cad'+'et'='CADET'
(Other definitions for cadet that I've seen before include "recruit" , "who needs to be properly trained?" , "Officer-trainee" , "Apprentice officer" , "Officer in training" .)