Principal structure (4)
I believe the answer is:
arch
This clue is a double definition.
'principal' is the first definition.
(as in arch-enemy)
'structure' is the second definition.
(arch is a kind of structure)
(Other definitions for arch that I've seen before include "Curved vault" , "Playfully superior" , "Teasing; part of foot" , "Curvature" , "Way though - on foot!" .)