Esoteric source of classic No. 1? Not hard to be repulsed (7)

I believe the answer is:

gnostic

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'esoteric' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?

I can't explain the remainder of the clue.

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(Other definitions for gnostic that I've seen before include "Relating to mystical knowledge" , "Having special mystical knowledge, costing a lot" , "Occult" .)

I've seen this clue in The Guardian.

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