Where rivers run fast (6)
I believe the answer is:
rapids
'where rivers run fast' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for rapids that I've seen before include "Fast flowing stretch of a river" , "Current hazard" , "fast flowing water" , "Part of a river where the current is very fast" , "Fast-flowing stretch of river, white water" .)