Sport played with mallets (4)
'sport played with mallets' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for polo that I've seen before include "Sweet" , "Game played on horses or in water" , "Game played on horseback wielding mallets" , "jersey" , "Pool (anag.)" .)
(Other definitions for croquet that I've seen before include "Outdoor game" , "Game played with mallets (or flamingoes according to Lewis Carroll)" , "Game with balls and mallet" , "genteel sport" , "Lawn sport" .)