Something afoot of an open nature (6)

I believe the answer is:

sandal

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'something afoot' is the definition.
(whimsically, something worn on a foot)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for sandal that I've seen before include "Light open-toed shoe" , "Item of footwear" , "Light open shoe" , "An open shoe for summer" , "One can go on foot" .)

I've seen this clue in the Evening Standard.

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