Crossword Genius

The world's greatest swimmer (4,5)

Ross

I believe the answer is:

blue whale

I'm a little stuck... Click here to teach me more about this clue!

This clue might be a double definition.

'the world's' is the first definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

'greatest swimmer' is the second definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

Can you help me to learn more?

(Other definitions for blue whale that I've seen before include "World's largest animal" , "World's largest mammal" , "Largest living animal" , "there's nothing bigger" .)

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