Flower arrangement for master (6)
I believe the answer is:
stream
'flower' is the definition.
(I know that stream is a type of river)
'arrangement for master' is the wordplay.
'arrangement for' is an anagram indicator (I've seen this in other clues).
'master' is an anagram of 'STREAM'.
(Other definitions for stream that I've seen before include "River - current" , "Natural body of running water" , "divide pupils into ability groups" , "Flow of water on land" , "Brook, burn" .)