River race (4)
I believe the answer is:
dart
This clue is a double definition.
'river' is the first definition.
(River Dart in Devon)
'race' is the second definition.
(I know that dart is a type of race)
(Other definitions for dart that I've seen before include "Run suddenly" , "Small spiked missile thrown in sport" , "Sudden quick movement" , "Arrow - go fast" , "One fired" .)