River mammal going after river rat! (6)
I believe the answer is:
rotter
'rat' is the definition.
('rotter' can be a synonym of 'rat')
'river mammal going after river' is the wordplay.
'river mammal' becomes 'otter' (I've seen this before).
'going after' means one lot of letters go next to another.
'river' becomes 'r'.
'otter' after 'r' is 'ROTTER'.
(Other definitions for rotter that I've seen before include "Pig" , "Something going off" , "Cad, bounder" , "one going bad?" , "Bad lot" .)