Fast, as a way to get fit, funnily enough! (5)

I believe the answer is:

swift

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'fast' is the definition.
(similar in meaning)

I cannot understand how the rest of the clue works.

(Other definitions for swift that I've seen before include "Quick - writer" , "Speedy - bird like a swallow" , "Expeditious" , "Anglo-Irish satirist" , "Fast; bird; writer" .)

I've seen this clue in the Evening Standard.

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