Polecat (6)
I believe the answer is:
ferret
'polecat' is the definition.
(ferret is a kind of polecat)
This is all the clue.
(Other definitions for ferret that I've seen before include "Animal used to catch rabbits" , "Animal trained to catch rabbits" , "Polecat bred for hunting rats and rabbits" , "Small fierce animal used to hunt rabbits" , "Assiduous searcher" .)