Unacceptable idea I rejected (3,2)
I believe the answer is:
not on
'unacceptable' is the definition.
(something 'not on' is unacceptable or frowned upon)
'idea i rejected' is the wordplay.
'idea' becomes 'notion' (notion is a kind of idea).
'rejected' 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 "It's unfeasible" , "Informally, cannot be permitted or tolerated" , "Unacceptable (colloq.)" , "Out of the question" , "Informally, can't be accepted or tolerated" .)