Oven with a halo round it when working? (4)
I believe the answer is:
oast
'oven' is the definition.
(type of kiln)
'halo round it when working?' is the wordplay.
I cannot really see how this works, but
'halo' could be 'o' (O resembles a halo) and 'o' is found within the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
'when' could be 'as' (eg 'when I walked' means 'as I walked') and 'as' is found within the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'with a' acts as a link.
Can you help me to learn more?
(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" .)