How to march on foot (6)

I believe the answer is:

instep

I'm a little stuck... Click here to teach me more about this clue!

'foot' is the definition.
(thesaurus)

I don't understand how the remainder of the clue works.

(Other definitions for instep that I've seen before include "Sir Jack --, famous English cricketer" , "Foot arch - conforming" , "Arch we stand on" , "Central part of a foot" , "Marching together correctly" .)

I've seen this clue in The Globe and Mail and The Irish Independent.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.