Costume on time for event (8)
I believe the answer is:
dressage
'for event' is the definition.
(I've seen this before)
'costume on time' is the wordplay.
'costume' becomes 'dress' (I've seen this in other clues).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'time' becomes 'age' (an age is a period of time).
'dress'+'age'='DRESSAGE'
(Other definitions for dressage that I've seen before include "equestrian activity" , "Horse-training" , "Equestrian performance" , "Equestrian manoeuvres" , "discipline for horses" .)