First part of a three-day equestrian event — reeds sag (anag) (8)
I believe the answer is:
dressage
(Other definitions for dressage that I've seen before include "The training of a horse in manoeuvres" , "Method of training horses to do manoeuvres" , "Art of horse training" , "Equestrian discipline" , "Manoeuvres of a horse in response to body signals by its rider" .)