Dog ruined river tree (9)
I believe the answer is:
retriever
'dog' is the definition.
(retriever is a kind of dog)
'ruined river tree' is the wordplay.
'ruined' indicates an anagram.
'river'+'tree'='rivertree'
'rivertree' with letters rearranged gives 'RETRIEVER'.
(Other definitions for retriever that I've seen before include "Breed of dog" , "Barker" , "He makes good" , "He brings back" , "Dog bred to fetch game" .)