Run quickly (4)
I believe the answer is:
hare
'run quickly' is the definition.
(hare can mean to run fast)
This is all the clue.
(Other definitions for hare that I've seen before include "Leveret" , "Fast-running mammal" , "As mad as this March animal" , "'You can't run with the . . . . and hunt with the hounds'" , "Mammal - run fast" .)