Tog up to get on in equestrian discipline (8)
I believe the answer is:
dressage
'equestrian discipline' is the definition.
(I've seen this before)
'tog up to get on' is the wordplay.
'tog up' becomes 'dress' (dressing is a kind of togging up**).
'to get on' becomes 'age' ('age' can be a synonym of 'get on').
'dress'+'age'='DRESSAGE'
'in' is the link.
(Other definitions for dressage that I've seen before include "equestrian routine" , "Method of horse training" , "Equestrian performance" , "Manoeuvres of a horse in response to body signals by its rider" , "The training of a horse in manoeuvres" .)