Brown bread, unopened oven (4)
I believe the answer is:
oast
'oven' is the definition.
(type of kiln)
'brown bread unopened' is the wordplay.
'brown bread' becomes 'toast' (I've seen this before).
'unopened' means to remove the first letter (without opening letter).
'toast' with its first letter taken off is 'OAST'.
(Other definitions for oast that I've seen before include "Hop drying kiln" , "Drying house" , "Kiln for drying hops or malt" , "Where to cook" , "Cooker" .)