Unacceptable, even after being overturned (3,2)
I believe the answer is:
not on
'unacceptable' is the definition.
(something 'not on' is unacceptable or frowned upon)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for not on that I've seen before include "Informally, cannot be permitted or tolerated" , "Common expression meaning won't be entertained or tolerated" , "Informally, can't be accepted or tolerated" , "Impractical" , "Colloquially, cannot be tolerated or accepted" .)