Trendy river animal (5)
I believe the answer is:
hippo
'animal' is the definition.
(I've seen this before)
'trendy river' is the wordplay.
'trendy' becomes 'hip' (I've seen this in another clue).
'river' becomes 'Po' (Italian river).
'hip'+'po'='HIPPO'
(Other definitions for hippo that I've seen before include "One thick-skinned" , "Augustine looked after" , "Tropical African mud wallower" , "Water-loving mammal" , "one that's large and fleshy" .)