Crossword Genius

Overweight yet fast, not a slow starter (3)

Ross

I believe the answer is:

fat

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'overweight' is the definition.
(similar in meaning)

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for fat that I've seen before include "Portly" , "Obese, having spare tyre etc" , "Greasy substance" , "As Bunter" , "Like 10 ac" .)

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