Principal part of foot (4)
I believe the answer is:
arch
I believe this is a double definition.
'principal' is the first definition.
(as in arch-enemy)
'part of foot' is the second definition.
(I've seen this before)
(Other definitions for arch that I've seen before include "Devilish" , "part of fingerprint" , "Curved entrance" , "Curved bony structure in the foot" , "Prefix meaning cheif, perhaps enemy" .)