Cunning, avoiding start of demonstration (4)

I believe the answer is:

arch

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'cunning' is the definition.
(arch can mean cunning or tricksy)

I cannot understand how the rest of the clue works.

(Other definitions for arch that I've seen before include "Curved masonry construction" , "Curved aperture" , "Curved span" , "Vaulted opening" , "Leading figures" .)

I've seen this clue in The Times.

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