Structure with keystone (4)
I believe the answer is:
arch
I believe this clue is a double definition.
'structure' is the first definition.
(arch is a kind of structure)
'keystone' is the second definition.
(thesaurus)
'with' acts as a link.
(Other definitions for arch that I've seen before include "in short, is out of date" , "Masonry structure for spanning an opening" , "Masonry construction for spanning an opening" , "Knowing, coy" , "Instep of foot" .)