Hop-drying kiln (4)
I believe the answer is:
'hop-drying kiln' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for oast that I've seen before include "Kiln for drying hops or malt" , "Cooker" , "Hop-drying kiln" , "Hop-drier" , "Hop-kiln" .)