Not on (3)
'not on' is the definition.
('off' is the opposite to 'on')
This is all the clue.
(Other definitions for off that I've seen before include "No longer available" , "Away, or no longer on the menu perhaps" , "Away, at a distance" , "Rotten" , "(Of food) no longer fresh" .)
'not on' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for unacceptable that I've seen before include "Not satisfactory" , "Not allowable" .)