One pulling a fast one, did you say? That's very quick (7)
I believe the answer is:
cheetah
'one pulling a fast one did you say? that's very quick' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
This is the entire clue.
Can you help me to learn more?
(Other definitions for cheetah that I've seen before include "Very fast mammal" , "Fast-running creature" , "Member of cat family" , "Animal that's remarkably quick" , "Fast predator" .)