Fast part of a river ride (6)
I believe the answer is:
rapids
'fast part of a river ride' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rapids that I've seen before include "Fast-flowing and turbulent part of a river" , "fast flowing water" , "Current hazard" , "White water area" , "Parts of a river where the current is very fast" .)