Crossword Genius

You need room to swing one (3)

Ross

I believe the answer is:

cat

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'one' is the definition.
(thesaurus)

I cannot understand how the rest of the clue works.

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(Other definitions for cat that I've seen before include "Animal; converter; boat" , "Feline - whip" , "An old whip with nine tails" , "Feline mammel" , "Animal - one with nine tails!" .)

I've seen this clue in the Evening Standard.
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