Using only one leg, it has little horse-power! (7)

I believe the answer is:

scooter

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'using' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.

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

Can you help me to learn more?

(Other definitions for scooter that I've seen before include "Light motorcycle" , "Motorised bike" , "Large diving duck" , "Moveable toy" , "Small motorcycle" .)

I've seen this clue in the Evening Standard.

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