Turbulent part of a river (6)
(Other definitions for rapids that I've seen before include "Fast-flowing and turbulent part of a river" , "Fast-flowing stretch of river, white water" , "Current hazard" , "what intrepid canoeist enjoys" , "Fast-flowing river stretches, white water" .)
(Other definitions for rapid that I've seen before include "Fast, quick" , "Zippy" , "Happening quickly" , "Speedy (5)" , "Brisk" .)