Clare's so playing the game (8)
I believe the answer is:
lacrosse
'the game' is the definition.
(lacrosse is a kind of game)
'clare's so playing' is the wordplay.
'playing' indicates an anagram.
'clares'+'so'='claresso'
'claresso' is an anagram of 'LACROSSE'.
(Other definitions for lacrosse that I've seen before include "Ball game using netted stick" , "Game played with nets on sticks" , "Score's uncertain after the Spanish" , "Game played with netted sticks" , "Sport played with sticks" .)