Brewer's kiln
I believe the answer is:
'brewer's 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 "Oven for hops" , "Hop drying kiln" , "Kiln for drying hops or malt" , "Drying house" , "Cooker" .)