Equestrian discipline (8)
I believe the answer is:
dressage
'equestrian discipline' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for dressage that I've seen before include "The training of a horse in manoeuvres" , "equestrian activity" , "Red gases (anag)" , "Part of equestrian event" , "Olympic event" .)