Naturally, a heated outburst from down under (8)

I believe the answer is:

eruption

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'down under' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Maybe they are linked in a way I don't understand?

I don't understand how the rest of the clue works.

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(Other definitions for eruption that I've seen before include "A violent breaking out, as of volcano" , "Breaking out (of spots, lava etc)" , "which blighted Pompeii?" , "Volcanic explosion" , "Volcanic outburst" .)

I've seen this clue in the Evening Standard.

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