Inner side of the foot (4)
I believe the answer is:
arch
'inner side of the foot' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for arch that I've seen before include "Affectedly playful or extreme" , "Curved masonry construction" , "Foremost" , "Instep of foot" , "Curve of a bridge" .)