I believe the answer is:
'unacceptable' is the definition.
(something 'not on' is unacceptable or frowned upon)
This is all the clue.
(Other definitions for not on that I've seen before include "Unacceptable (colloq.)" , "Definitely inadvisable" , "Informally, cannot be permitted or tolerated" , "Impractical" , "Modern usage for unacceptable" .)