How soldiers march, on foot (6)

I believe the answer is:

instep

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'foot' is the definition.
(thesaurus)

I cannot understand how the remainder of the clue works.

(Other definitions for instep that I've seen before include "Part of foot/shoe" , "Upper surface of the foot" , "Part of foot (how to march!)" , "Sir Jack --, famous English cricketer" , "Part of foot between toes and ankle" .)

I've seen this clue in the Evening Standard.

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