Flower of the master? (6)
I believe the answer is:
stream
'flower' is the definition.
(I know that stream is a type of river)
'of the master?' is the wordplay.
'of' indicates anagramming the letters (the answer comes from or 'of' the letters).
'master' is an anagram of 'STREAM'.
(Other definitions for stream that I've seen before include "Current - file" , "Small narrow flow" , "Brook or small river" , "Flow of water on land" , "class division" .)