Return students make mistakes (4,2)
I believe the answer is:
slip up
'return students make mistakes' is the definition.
I can't tell whether this definition defines the answer.
'return students' is the wordplay.
'return' says the letters should be written backwards.
'students' becomes 'pupils' ('pupil' can be a synonym of 'student').
'pupils' in reverse letter order is 'SLIP-UP'.
Can you help me to learn more?
(Other definitions for slip up that I've seen before include "Make a mistake, being unsteady on feet?" , "Mistake, blunder" , "Minor inadvertent mistake" , "get it wrong" , "Make an error" .)