It's unacceptable either way (3,2)
I believe the answer is:
not on
'it's unacceptable' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for not on that I've seen before include "Colloquially, cannot be tolerated or accepted" , "Informally, cannot be permitted or tolerated" , "Common expression meaning won't be entertained or tolerated" , "Impractical" , "Modern phrase for unacceptable" .)