Knowing what may be Norman (4)

I believe the answer is:

arch

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'knowing' is the definition.
(I've seen this before)

I can't explain the remainder of the clue.

(Other definitions for arch that I've seen before include "Curve that spans an opening" , "part of fingerprint" , "Supporting structure" , "Curved form" , "one under a foot?" .)

I've seen this clue in the Evening Standard.

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