Round microwave? (4)
I believe the answer is:
heat
I believe this clue is a double definition.
'round' is the first definition.
(stage of a competition)
'microwave?' is the second definition.
(I've seen this in another clue)
(Other definitions for heat that I've seen before include "Energy that causes a rise in temperature" , "Energy that caues a rise in temperature" , "Apply warmth" , "Hotness" , "Vehemence" .)