Do the wrong thing in reverting to form, perhaps (4,2)
I believe the answer is:
slip up
'do the wrong thing in reverting to form perhaps' is the definition.
I can't judge whether this defines the answer.
'reverting to form perhaps' is the wordplay.
'reverting' is a reversal indicator.
'to form perhaps' becomes 'pupils' (I can't explain this - if you can you should believe this answer much more).
'pupils' reversed gives 'SLIP-UP'.
Can you help me to learn more?
(Other definitions for slip up that I've seen before include "Maybe literal" , "Minor lapse" , "err" , "get it wrong" , "Make a mistake or lose footing aloft" .)