White water (6)
I believe the answer is:
'white water' 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 "Where rivers run fast" , "Part of a river where the current is very fast" , "what intrepid canoeist enjoys" , "Current hazard" , "Fast-flowing river stretches, white water" .)