Pig or snake seen by river (4)
I believe the answer is:
boar
'pig' is the definition.
(male pig)
'snake seen by river' is the wordplay.
'snake' becomes 'boa' (boa constrictor, type of snake).
'seen by' is a charade indicator (letters next to each other).
'river' becomes 'r' (abbreviation for river).
'boa'+'r'='BOAR'
'or' is the link.
(Other definitions for boar that I've seen before include "Male swine" , "Wild pig sounds tedious" , "A wild male pig" , "Resting place for birds or bats" , "Uncastrated male pig" .)