River creature going after river rat (6)
I believe the answer is:
rotter
'rat' is the definition.
(I know that rat can be written as rotter)
'river creature going after river' is the wordplay.
'river creature' becomes 'otter' (I've seen this before).
'going after' means one lot of letters go next to another.
'river' becomes 'r' (abbreviation for river).
'otter' put after 'r' is 'ROTTER'.
(Other definitions for rotter that I've seen before include "One decomposing" , "One not to be honoured" , "An informal cad, may be decaying" , "Bad guy" , "Cad, stinker" .)