Game played on horses or in water (4)
I believe the answer is:
polo
'game played on horses or in water' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for polo that I've seen before include "Game played on horses or in water" , "Game played in chukkas" , "Horse-game played with loop" , "Pool (anag.)" , "jersey" .)