Chief part of the foot (4)
I believe the answer is:
arch
I believe this is a double definition.
'chief' is the first definition.
('arch' can be a synonym of 'chief')
'part of the foot' is the second definition.
(I've seen this before)
(Other definitions for arch that I've seen before include "Waggish" , "Prefix meaning cheif, perhaps enemy" , "Shrewd; bridge" , "Foremost" , "Way though - on foot!" .)