Backing students a mistake? (4-2)
I believe the answer is:
slip-up
'a mistake' is the definition.
(I've seen this before)
'backing students' is the wordplay.
'backing' shows that the letters should be reversed in order.
'students' becomes 'pupils' ('pupil' can be a synonym of 'student').
'pupils' back-to-front is 'SLIP-UP'.
(Other definitions for slip-up that I've seen before include "Minor lapse" , "Minor inadvertent mistake" , "Mistake, blunder" , "Small error" .)