Tortoise's opponent (4)
I believe the answer is:
hare
'tortoise's opponent' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for hare that I've seen before include "Rabbit-like animal" , "Mammal like a large rabbit" , "Timid creature" , "Game, -- and hounds" , "Old saying: 'Let the .... sit" .)