Crossword Genius

I must block unacceptable idea (6)

Ross

I believe the answer is:

notion

'idea' is the definition.
(notion is a kind of idea)

'i must block unacceptable' is the wordplay.
'must block' is an insertion indicator (inserted letters block or plug up a gap).
'unacceptable' becomes 'not on' (something 'not on' is unacceptable or frowned upon).
'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" , "Idea or fancy" , "desire" , "Vague or whimsical idea" .)

I've seen this clue in The Times.
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