Unacceptable to pinch one’s idea (6)
I believe the answer is:
notion
'idea' is the definition.
(I know that idea can be written as notion)
'unacceptable to pinch one's' is the wordplay.
'unacceptable' becomes 'not on' (something 'not on' is unacceptable or frowned upon).
'to pinch' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'noton' going around 'i' is 'NOTION'.
(Other definitions for notion that I've seen before include "An idea or casual thought" , "Vague or whimsical idea" , "Vague idea or opinion" , "Vague understanding" , "Vague impression" .)