Marco ____ (4)
I believe the answer is:
polo
'marco' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for polo that I've seen before include "Game on horseback or in water" , "What riders might get up to in a team" , "Horsey ball game" , "Game - Venetian traveller" , "Pool (anag.)" .)