Student's set back by small mistakes (4-3)
I believe the answer is:
Here is my best explanation:
'mistakes' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'student's set back by small' is the wordplay.
'student's' becomes 'pupils' ('pupil' can be a synonym of 'student').
'set back' says the letters should be written in reverse.
'by' says to put letters next to each other.
'small' becomes 's' (abbreviation - e.g. clothes size).
'pupils' back-to-front is 'slipup'.