Cad finds river animal (6)
I believe the answer is:
rotter
'cad' is the definition.
(thesaurus)
'river animal' is the wordplay.
'river' becomes 'r'.
'animal' becomes 'otter' (I've seen this before**).
'r'+'otter'='ROTTER'
'finds' is the link.
(Other definitions for rotter that I've seen before include "Bad guy" , "Absolute bounder" , "Bad lot" , "one going bad?" , "Scoundrel" .)