It's on foot, though a horse is involved (7)

I believe the answer is:

pastern

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If this is the answer, I'm afraid I cannot explain it.

(Other definitions for pastern that I've seen before include "Part of horse's foot" , "'The knee of an horse' (Johnson)" , "Fetter-bone of horse" , "Wrongly-defined part of horse" , "part of Arab's leg" .)

I've seen this clue in the Evening Standard.

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