With costume, time for equestrian event (8)
I believe the answer is:
dressage
'equestrian event' is the definition.
(I've seen this in another clue)
'with costume time' is the wordplay.
'with' means one lot of letters go next to another.
'costume' becomes 'dress' (I've seen this in other clues).
'time' becomes 'age' (an age is a period of time).
'dress'+'age' is 'DRESSAGE'.
'for' acts as a link.
(Other definitions for dressage that I've seen before include "Mounted training" , "Training for competition" , "Equestrian sport of manoeuvring" , "Riding skill" , "Art of training horses" .)