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' indicates named letters should be taken away.
'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" .)