Equestrian game (4)
I believe the answer is:
polo
'equestrian game' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for polo that I've seen before include "car?" , "explorer of the thirteenth century" , "In which horsemen attack and defend" , "It's played in water and on horseback" , "Venetian 19 to Asia, servant to Kublai Khan" .)