Equestrian competition (5-3,5)
I believe the answer is:
three-day event
'equestrian competition' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for three-day event that I've seen before include "Dressage, cross-country and showjumping" , "Equestrian competition comprising several disciplines" , "Equestrian competition involving several skills" , "Horse-riding competition" .)