Bone found by river beast (5)
I believe the answer is:
hippo
'beast' is the definition.
(I've seen this before)
'bone found by river' is the wordplay.
'bone' becomes 'hip'.
'found by' means one lot of letters go next to another.
'river' becomes 'Po' (Italian river).
'hip'+'po'='HIPPO'
(Other definitions for hippo that I've seen before include "Large animal" , "Mammoth African mammal (abbr)" , "Water-loving mammal" , "Wallowing mammal" , "Afr. river mammal" .)