Top sport (4)
I believe the answer is:
polo
I believe this clue is a double definition.
'top' is the first definition.
(polo top)
'sport' is the second definition.
(polo is a kind of sport)
(Other definitions for polo that I've seen before include "Game played on horses or in water" , "car?" , "jersey" , "Game (with a hole?)" , "Game on horseback or in water" .)