Curved entranceway (4)
I believe the answer is:
arch
'curved entranceway' 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 "Curve spanning an opening" , "Mischievous - shrewd" , "Bridge span" , "Self-consciously playful or teasing" , "Chief - opening with curved top" .)