One who wangles tip from passenger in taxi? Animal! (7,4)

I believe the answer is:

fiddler crab

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This clue could be a double definition.

'one who wangles tip from passenger in taxi?' is the first definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.

'animal' is the second definition.
(fiddler crab is a kind of animal)

Can you help me to learn more?

(Another definition for fiddler crab that I've seen is " Small crustacean".)

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