Brown mature river bird (7)
I believe the answer is:
tanager
'bird' is the definition.
(tanager is a kind of bird)
'brown mature river' is the wordplay.
'brown' becomes 'tan' (tan is a kind of brown).
'mature' becomes 'age' (synonyms).
'river' becomes 'r' (abbreviation).
'tan'+'age'+'r'='TANAGER'
(Other definitions for tanager that I've seen before include "South American singer" , "A bird" , "Songbird of the bunting family" , "American songbird" .)