How the rival merits of electric fires were discussed? (8)

I believe the answer is:

heatedly

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'how' is the definition.
Both the answer and definition are adverbs. Perhaps you can see a link between them that I can't see?

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(Other definitions for heatedly that I've seen before include "How people might argue" , "With passion" , "Angrily, being warm?" , "In an emotional and inflamed manner" .)

I've seen this clue in the Evening Standard.

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