Tortoise’s rival? (4)
I believe the answer is:
hare
'tortoise's rival?' 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 "Swift mammal" , "Long-eared mammal" , "Run with great speed" , "'Swift, long-eared animal (4)'" , "Small rabbit-like mammal" .)