Aquatic sport (5,4)
'aquatic sport' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for water polo that I've seen before include "Game played by tams of swimmers with large ball (5.4)" , "Seven-a-side game involving swimming" , "Team sport with wet participants" , "Game played by swimming teams with large ball" , "Game played by teams of swimmers with large ball" .)
(Other definitions for diving that I've seen before include "Plunging headlong" , "Plunging oneself into water" , "Going underwater" , "Going under water" , "water event" .)