Black variety of rat (3)
I believe the answer is:
tar
'black' is the definition.
(I've seen this in other clues)
'variety of rat' is the wordplay.
'variety of' indicates anagramming the letters.
'rat' with letters rearranged gives 'TAR'.
(Other definitions for tar that I've seen before include "Dark viscous stuff" , "Viscous bituminous substance" , "Road surfacing or old sailor" , "Product of burning tobacco" , "Product of smoking cigarettes" .)