Idiot holding back star pupil? On the contrary (4)
I believe the answer is:
prat
'idiot' is the definition.
(both can mean a stupid person)
'holding back star pupil? on the contrary' is the wordplay.
'holding' says the answer is hidden in the clue.
'on the contrary' shows that the letters should be reversed in order (letters in the opposite or contrary direction).
'tarp' is hidden in the letters of 'back star pupil'.
'tarp' backwards is 'PRAT'.
(Other definitions for prat that I've seen before include "Rapt like silly ass" , "In slang, a fool" , "Rapt kind of fool" , "Incompetent idiot" , "Wally" .)