Structure with a keystone (4)
I believe the answer is:
arch
'structure with a keystone' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for arch that I've seen before include "Curved part of stone bridge, perhaps" , "Instep of foot" , "Clever" , "Roguish, teasing" , "Cunning; span" .)