Animal in cool river (5)
I believe the answer is:
hippo
'animal' is the definition.
(I've seen this before)
'cool river' is the wordplay.
'cool' becomes 'hip'.
'river' becomes 'po' (Po is an example).
'hip'+'po'='HIPPO'
'in' acts as a link.
(Other definitions for hippo that I've seen before include "Large semi-aquatic pachyderm (abbr)" , "Aquatic mammal" , "African animal (shortened)" , "22 aquatic animal" , "Mammoth African mammal (abbr)" .)