Chief curved structure (4)
I believe the answer is:
arch
I believe this is a double definition.
'chief' is the first definition.
(synonyms)
'curved structure' is the second definition.
(architectural structure)
(Other definitions for arch that I've seen before include "Curved aperture" , "Playfully superior" , "Chief - opening with curved top" , "Marble, for example" , "Curved overhead structure" .)