Principal gateway perhaps (4)
I believe the answer is:
arch
I believe this clue is a double definition.
'principal' is the first definition.
(thesaurus)
'gateway perhaps' is the second definition.
(Other definitions for arch that I've seen before include "Curved masonry construction" , "Knowing, coy" , "Shrewd; bridge" , "Bridge span" , "Annoyingly playful" .)