Overweight yet fast, not a slow starter (3)
I believe the answer is:
fat
'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" .)