Like a march or an arch? (6)

I believe the answer is:

instep

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

'an arch?' is the definition.
(I know that instep is a type of arch)

I can't explain the rest of the clue.

(Other definitions for instep that I've seen before include "Central part of a foot" , "Part of foot under spinet" , "Part of foot or shoe" , "Foot arch - conforming" , "Arch area of the foot" .)

I've seen this clue in The Independent.

Want a hint initially instead of a full solution? Install my app