Unacceptable (3,2)
I believe the answer is:
not on
'unacceptable' is the definition.
(something 'not on' is unacceptable or frowned upon)
This is the entire 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" .)