Oven with a halo round it when working? (4)

I believe the answer is:

oast

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'oven' is the definition.
(oast is a kind of oven)

I cannot understand how the remainder of the clue works.

(Other definitions for oast that I've seen before include "Hop-drying kiln" , "Drying house" , "Kiln for drying hops or malt" , "Hop-kiln" , "Oven for hops" .)

I've seen this clue in the Evening Standard.

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