Sport with horses (4)
I believe the answer is:
polo
'sport with horses' 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, anagram of ..." , "Game played on horseback wielding mallets" , "Sport - mint - explorer" , "explorer of the thirteenth century" , "A mint on horseback?" .)