Mouse's kin (3)
I believe the answer is:
rat
'mouse's kin' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rat that I've seen before include "Treacherous person" , "one not going down with ship" , "Unpopular rodent" , "Despicable person" , "Tell-tale" .)