Ring-road goes round a kiln (4)
I believe the answer is:
oast
'kiln' is the definition.
(oast is a kind of kiln)
'ring-road goes round a' is the wordplay.
'ring' becomes 'o'.
'road' becomes 'st'.
'goes round' indicates putting letters inside.
'o'+'st'='ost'
'ost' enclosing 'a' is 'OAST'.
(Other definitions for oast that I've seen before include "Hop drying kiln" , "Hop-drier" , "Kiln for drying hops or malt" , "Drying house" , "Cooker" .)