One taken in by unacceptable idea (6)
I believe the answer is:
notion
'idea' is the definition.
(notion is a kind of idea)
'one taken in by unacceptable' is the wordplay.
'one' becomes 'i' (Roman numeral).
'taken in by' is an insertion indicator.
'unacceptable' becomes 'noton' (I've seen this before).
'i' put into 'noton' is 'NOTION'.
(Other definitions for notion that I've seen before include "Vague or fanciful idea" , "Vague idea or opinion" , "Whimsical idea or small sewing article" , "Idea, belief" , "clue" .)