Tortoise rival (4)
I believe the answer is:
hare
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">tortoise rival</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for hare that I've seen before include "Swift mammal" , "Small rabbit-like mammal" , "may be electric!" , "Swift rodent" , "Sect: -- Krishna" .)