Unacceptable idea I ignored (3,2)
I believe the answer is:
not on
'unacceptable' is the definition.
(something 'not on' is unacceptable or frowned upon)
'idea i ignored' is the wordplay.
'idea' becomes 'notion' (notion is a kind of idea).
'ignored' is a deletion indicator.
'notion' with 'i' taken away is 'NOT ON'.
(Other definitions for not on that I've seen before include "Informally, can't be accepted or tolerated" , "taboo" , "Common expression meaning won't be entertained or tolerated" , "Modern phrase for unacceptable" , "Improper - impossible" .)