A period of play in polo (6)
I believe the answer is:
chukka
'a period of play in polo' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for chukka that I've seen before include "7 1/2 minutes of polo" , "period of game" , "Part of a polo game" , "Polo game division" , "Period of play in a polo match" .)