Groom on time in equestrian event (8)
I believe the answer is:
dressage
'equestrian event' is the definition.
(I've seen this in another clue)
'groom on time' is the wordplay.
'groom' becomes 'dress' (dressing is a kind of grooming).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'time' becomes 'age' (an age is a period of time).
'dress'+'age'='DRESSAGE'
'in' is the link.
(Other definitions for dressage that I've seen before include "equestrian routine" , "Equestrian sport of manoeuvring" , "Olympic event" , "Training for competition" , "Art of training horses" .)