Top off stone entrance (4)
I believe the answer is:
gate
'entrance' is the definition.
(type of entrance)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for gate that I've seen before include "Entrance (money)" , "Kind of door as in wall or fence" , "Field entrance" , "Plane-boarding point" , "Portal" .)