Principal part of the foot (4)
I believe the answer is:
arch
I believe this clue is a double definition.
'principal' is the first definition.
(as in arch-enemy)
'part of the foot' is the second definition.
(I've seen this before)
(Other definitions for arch that I've seen before include "Construction with a keystone" , "Shrewd; bridge" , "spanner" , "Curved gateway" , "Chief - opening with curved top" .)