How to accept a unique opportunity (4)

I believe the answer is:

once

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

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

Can you help me to learn more?

(Other definitions for once that I've seen before include "On one occasion" , "Formerly - not repeated" , "Just as a single occurrence" , "yesterday" , "Long ago" .)

I've seen this clue in the Evening Standard.

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