Aesop racer (4)
I believe the answer is:
hare
'aesop racer' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for hare that I've seen before include "Fast-running mammal" , "As mad as this March animal" , "Game, -- and hounds" , "Mammal - run fast" , "Rabbit-like animal" .)