Sport played on horseback (4)
I believe the answer is:
polo
'sport played 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 "Game played on horseback wielding mallets" , "Game; Italian explorer" , "Sport - mint - explorer" , "Game; Venetian explorer" , "Mounted game" .)