Master arranged flower (6)
I believe the answer is:
stream
'flower' is the definition.
(I know that stream is a type of river)
'master arranged' is the wordplay.
'arranged' is an anagram indicator (letters rearranged).
'master' anagrammed gives 'STREAM'.
(Other definitions for stream that I've seen before include "'Current, flow (6)'" , "Constant flow of liquid" , "pupils pour out" , "Torrent" , "Master of a tributary" .)