Some pupil sneakily raised error (4)
I believe the answer is:
slip
'error' is the definition.
(slip is a kind of error)
'some pupil sneakily raised' is the wordplay.
'some' indicates a hidden word (some parts of the word are used).
'raised' says the letters should be written in reverse (going up in a down clue).
'pils' is hidden within 'pupil sneakily'.
'pils' in reverse letter order is 'SLIP'.
(Other definitions for slip that I've seen before include "shift" , "Faulty" , "Gaffe" , "Clay, water mix; fielder" , "He should take his chances" .)