Crossword Genius

How speedily you demonstrated the torch was working? (2,1,5)

Ross

I believe the answer is:

in a flash

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

I cannot understand how the rest of the clue works.

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(Other definitions for in a flash that I've seen before include "How lightning appears" , "pictures may be taken" , "Very quickly" , "Instantly, straightaway" , "Almost instantly" .)

I've seen this clue in the Evening Standard.
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