One with a big share in a zoo? (4)

I believe the answer is:

lion

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'one' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

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

Can you help me to learn more?

(Other definitions for lion that I've seen before include "Big cat with pride" , "Courageous person" , "Carnivorous 6" , "Large cat; rugby player" , "Carnivore" .)

I've seen this clue in the Evening Standard.

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