One into unacceptable belief (6)
I believe the answer is:
notion
'belief' is the definition.
('notion' can be a synonym of 'belief')
'one into unacceptable' is the wordplay.
'one' becomes 'i' (Roman numeral).
'into' is an insertion indicator.
'unacceptable' becomes 'noton' (I've seen this before).
'i' placed within 'noton' is 'NOTION'.
(Other definitions for notion that I've seen before include "An idea or casual thought" , "Whimsical idea or small sewing article" , "desire" , "Vague or fanciful idea" , "Vague or whimsical idea" .)