Kind of arch (4)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
ogee
'kind of arch' is the definition.
This is all the clue.
(Other definitions for ogee that I've seen before include "Leer" , "S-shaped line" , "S-shaped moulding" , "architectural feature" , "bit of Gothic architecture?" .)