Principal feature of a vault (4)
I believe the answer is:
arch
I believe this clue is a triple definition!
'principal' is the first definition.
(as in arch-enemy)
'feature' is the second definition.
(I have seen 'architectural feature' mean 'arch' so perhaps 'feature' could also mean 'arch')
'of' acts as a link.
'a vault' is the third definition.
(I've seen this in another clue)
(Other definitions for arch that I've seen before include "Chief - supporter - cunning" , "one under a foot?" , "Affectedly playful (4)" , "Cunning; bridge" , "This enemy is major" .)