Unacceptable, going back and forth (3,2)
I believe the answer is:
not on
This clue is a double definition.
'unacceptable' is the first definition.
(something 'not on' is unacceptable or frowned upon)
'going back and forth' is the second definition.
(Other definitions for not on that I've seen before include "Unacceptable (colloq.)" , "Informally, can't be accepted or tolerated" , "Modern usage for unacceptable" , "Improper - impossible" , "Informally, cannot be permitted or tolerated" .)