Dog that races the bus? (9)
I believe the answer is:
greyhound
'dog' is the definition.
(greyhound is a kind of dog)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for greyhound that I've seen before include "Want to bet on it?" , "Fast-moving dog" , "Tall, slender canine" , "racer" , "Swift breed of dog" .)