Potentially ban peace cult? It's not on (12)
I believe the answer is:
unacceptable
'not on' is the definition.
(I've seen this before)
'potentially ban peace cult?' is the wordplay.
'potentially' indicates an anagram (letters have the potential to spell a new word).
'ban'+'peace'+'cult'='banpeacecult'
'banpeacecult' anagrammed gives 'UNACCEPTABLE'.
'it's' is the link.
(Other definitions for unacceptable that I've seen before include "Not satisfactory" , "Not allowable" .)