Section of foot soldiers marching must be this (6)

I believe the answer is:

instep

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'section of foot' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

(Other definitions for instep that I've seen before include "Part of foot under spinet" , "Central part of a foot" , "Arch area of the foot" , "One part of Oxford" , "Part of foot between toes and ankle" .)

I've seen this clue in The Times.

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