Horseback game (4)
I believe the answer is:
polo
'horseback game' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for polo that I've seen before include "Game played in chukkas" , "Sport - mint - explorer" , "Game played on horseback wielding mallets" , "Venetian 19 to Asia, servant to Kublai Khan" , "Horseback sport" .)