Younger son, rotter, extremely efficient (5)
I believe the answer is:
cadet
'younger son' is the definition.
(archaic term for a younger child)
'rotter extremely efficient' is the wordplay.
'rotter' becomes 'cad' (I've seen this in another clue).
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'efficient' with its centre taken out is 'et'.
'cad'+'et'='CADET'
(Other definitions for cadet that I've seen before include "Younger son's branch of a family" , "officer of the future" , "Police force trainee" , "recruit" , "Officer-trainee" .)