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