Working cat (6)
I believe the answer is:
mouser
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">working cat</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)