Snake and rodent let out by river (7)
I believe the answer is:
rattler
'snake' is the definition.
(rattler is a kind of snake)
'rodent let out by river' is the wordplay.
'rodent' becomes 'rat' (rat is a kind of rodent).
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'by' is a charade indicator (letters next to each other).
'river' becomes 'r'.
'let' anagrammed gives 'tle'.
'rat'+'tle'+'r'='RATTLER'
'and' is the link.
(Other definitions for rattler that I've seen before include "Poisonous American" , "North American pit viper" , "that makes one frightened" , "Nickname for a pit viper in the US" , "Venomous snake" .)