Fastest afloat? (5)

I believe the answer is:

fleet

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

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

Can you help me to learn more?

(Other definitions for fleet that I've seen before include "Marine force" , "Quickly pass" , "Swift; armada" , "Fast kind of armada?" , "Swift like navy" .)

I've seen this clue in the Evening Standard.

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