Shoot river into stream (6)
I believe the answer is:
sprout
'shoot' is the definition.
(I know that shoot can be written as sprout)
'river into stream' is the wordplay.
I cannot quite understand how this works, but
'river' could be 'r' (abbreviation for river) and 'r' is found in the answer.
'stream' could be 'po' (Po is an example) and 'po' is present in the leftover letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for sprout that I've seen before include "Grow like Proust" , "Suddenly grow" , "Bud; a type of cabbage" , "Put out shoots for a brassica" , "Put forth shoots" .)