Principal curve (4)
I believe the answer is:
arch
This clue is a double definition.
'principal' is the first definition.
(as in arch-enemy)
'curve' is the second definition.
(synonyms)
(Other definitions for arch that I've seen before include "Bridge span" , "Inner part of foot" , "part of fingerprint" , "Chief - supporter - cunning" , "Clever" .)