Curved gateway (4)
I believe the answer is:
arch
'curved gateway' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for arch that I've seen before include "Embow" , "one that may have fallen" , "Vaulted opening" , "Affectedly playful like curved structure" , "Vault; roguish" .)