Head of intelligence gripped by socially unacceptable thought (6)

I believe the answer is:

notion

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'thought' is the definition.
(I know that notion is a type of thought)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for notion that I've seen before include "Vague or fanciful idea" , "Caprice" , "Vague understanding" , "Idea or whim" , "Idea, concept" .)

I've seen this clue in The Guardian.

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