Working cat (6)
I believe the answer is:
mouser
'working cat' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for mouser that I've seen before include "Rodent catcher" , "Cat that's good at catching small rodents" , "Predatory cat" , "One getting rid of vermin" , "Preying cat" .)