One is entertained by unacceptable idea (6)
I believe the answer is:
notion
'idea' is the definition.
(I know that notion is a type of idea)
'one is entertained by unacceptable' is the wordplay.
'one' becomes 'i' (Roman numeral).
'is entertained by' indicates putting letters inside (entertain can mean to host or welcome in).
'unacceptable' becomes 'not on' (something 'not on' is unacceptable or frowned upon).
'i' inserted inside 'noton' is 'NOTION'.
(Other definitions for notion that I've seen before include "A vague and possibly incorrect opinion" , "Concept, belief" , "clue" , "An idea or casual thought" , "Caprice" .)