Rodent-catching cat (6)
I believe the answer is:
mouser
'rodent-catching cat' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for mouser that I've seen before include "Cat proficient at catching small rodents" , "Predatory cat" , "Cat as hunter of small rodents" , "Rodent catcher" , "Cat with certain skills" .)