Crossword Genius

Food critic scratching bottom quickly (6)

Ross

I believe the answer is:

presto

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'quickly' is the definition.
(I've seen this in another clue)

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

Can you help me to learn more?

(Other definitions for presto that I've seen before include "hey......,cries the magician" , "rapidly" , "quick to score" , "Moving quickly" , "Immediately" .)

I've seen this clue in the Sydney Morning Herald.
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