Principal supporting structure (4)
I believe the answer is:
arch
This clue is a double definition.
'principal' is the first definition.
(thesaurus)
'supporting structure' is the second definition.
(I've seen this before)
(Other definitions for arch that I've seen before include "Chief - supporter - cunning" , "Curved part of stone bridge, perhaps" , "This enemy is major" , "Curved span" , "Marble, for example" .)