The best runner (6)
I believe the answer is:
flower
This clue is a double definition.
'the best' is the first definition.
(flower can mean the best or elite)
'runner' is the second definition.
(a river flows)
(Other definitions for flower that I've seen before include "Blossom" , "One found in bed perhaps" , "rose, perhaps" , "bloomer" , "prime of life" .)