Game played on horseback (4)
I believe the answer is:
polo
'game played on horseback' 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; Venetian explorer" , "car?" , "It's played in water and on horseback" , "In which horsemen attack and defend" , "Sport - mint - explorer" .)