When reversing, learners will do the wrong thing (4,2)
I believe the answer is:
slip up
'will do the wrong thing' is the definition.
I can't judge whether this definition defines the answer.
'when reversing learners' is the wordplay.
'when reversing' says the letters should be written in reverse.
'learners' becomes 'pupils' (pupil is a kind of learner).
'pupils' backwards is 'SLIP-UP'.
Can you help me to learn more?
(Other definitions for slip up that I've seen before include "Make a mistake or lose footing aloft" , "Minor inadvertent mistake" , "Informally, make a mistake - lose footing?" , "Make a mistake, on a banana skin ?" , "one fielder close to the wicket?" .)