Cunning leader missing from demo (4)

I believe the answer is:

arch

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

I cannot understand how the rest of the clue works.

(Other definitions for arch that I've seen before include "Masonry construction for spanning an opening" , "Curved span" , "which may fall and inconvenience walkers" , "Curved gateway" , "Curved masonry construction" .)

I've seen this clue in The Times.

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