Crossword Genius

Twice reduced by 50% (4)

Ross

I believe the answer is:

once

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'twice' is the definition.
Both the definition and answer are adverbs. Perhaps there's a link between them I don't understand?

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

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(Other definitions for once that I've seen before include "Cone (anag.)" , "Just as a single occurrence" , "In times past" , "In former times" , "in bygone era" .)

I've seen this clue in The Globe and Mail.
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