Very hot oven in arena (8)
I believe the answer is:
roasting
'very hot' is the definition.
(I've seen this before)
'oven in arena' is the wordplay.
'oven' becomes 'oast' (type of kiln).
'in' is an insertion indicator.
'arena' becomes 'ring' (eg a boxing ring).
'oast' inserted within 'ring' is 'ROASTING'.
(Other definitions for roasting that I've seen before include "Cooking; severe criticism" , "Criticising" , "lecture" , "Lots of criticism" , "Very hot" .)