Like a march or an arch? (6)
I believe the answer is:
instep
'an arch?' is the definition.
(I know that instep is a type of arch)
I can't explain the rest of the clue.
Can you help me to learn more?
(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" .)