Unacceptable to hold one idea (6)
I believe the answer is:
notion
'idea' is the definition.
(notion is a kind of idea)
'unacceptable to hold one' is the wordplay.
'unacceptable' becomes 'noton' (I've seen this before).
'to hold' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'noton' placed around 'i' is 'NOTION'.
(Other definitions for notion that I've seen before include "Vague understanding" , "Vague idea or opinion" , "desire" , "Whim or fancy" , "Vague impression" .)