Horseback sport (4)
I believe the answer is:
polo
'horseback sport' 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 - Venetian traveller" , "Game played on horses or in water" , "Game; Venetian explorer" , "It's played in water and on horseback" , "Game; Italian explorer" .)