Move quickly in river (4)
I believe the answer is:
dart
I believe this is a double definition.
'move quickly' is the first definition.
(I've seen this before)
'river' is the second definition.
(River Dart in Devon)
'in' is the link.
(Other definitions for dart that I've seen before include "How a musketeer begins" , "Seasoned beef" , "flash" , "Small pointed projectile" , "one destined to go on board?" .)