That beastly joint over the river (5)
I believe the answer is:
hippo
'that beastly' is the definition.
I can't tell whether this definition defines the answer.
'joint over the river' is the wordplay.
'joint' becomes 'hip' (hip is a kind of joint).
'over' is a charade indicator (letters next to each other).
'the river' becomes 'Po' (Italian river).
'hip'+'po'='HIPPO'
Can you help me to learn more?
(Other definitions for hippo that I've seen before include "Mammoth African mammal (abbr)" , "one that's large and fleshy" , "Beastly sort" , "Large animal" , "African animal (shortened)" .)