Inner side of the foot (4)
I believe the answer is:
arch
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">inner side of the foot</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)