Fastest mammal (7)
I believe the answer is:
cheetah
'fastest mammal' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cheetah that I've seen before include "Fast cat - the ache (anag)" , "Swift-running cat" , "Long-legged spotted big cat" , "Animal that's remarkably quick" , "Large spotted wild cat, very fast" .)