Costume to get on for equestrian event (8)
I believe the answer is:
dressage
'equestrian event' is the definition.
(I've seen this in another clue)
'costume to get on' is the wordplay.
'costume' becomes 'dress' (I've seen this before).
'to get on' becomes 'age' ('age' can be a synonym of 'get on').
'dress'+'age'='DRESSAGE'
'for' is the link.
(Other definitions for dressage that I've seen before include "Training of a horse in deportment" , "Riding skill" , "Part of equestrian event" , "Red gases (anag)" , "Event for rider" .)