Arch of the foot (6)
I believe the answer is:
instep
'arch of the foot' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for instep that I've seen before include "Marching together correctly" , "Set pin in part of foot" , "Central part of a foot" , "Part of foot or shoe" , "downtrodden area" .)