Sport on horseback (4)
I believe the answer is:
polo
'sport on horseback' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for polo that I've seen before include "Team game played with horses" , "What riders might get up to in a team" , "Equestrian sport" , "Horseback sport" , "Ball game played on horseback" .)