An unacceptable palindrome (3,2)
I believe the answer is:
not on
I believe this clue is a double definition.
'an unacceptable' is the first definition.
(something 'not on' is unacceptable or frowned upon)
'palindrome' is the second definition.
(Other definitions for not on that I've seen before include "Out of the question" , "Impractical" , "Informally, can't be accepted or tolerated" , "Unacceptable (colloq.)" , "Informally, cannot be permitted or tolerated" .)