Wanting practice in game enclosure (5)
I believe the answer is:
rusty
'wanting practice' is the definition.
(rusty can mean out of practice)
'game enclosure' is the wordplay.
'game' becomes 'RU' (rugby union).
'enclosure' becomes 'sty' (sty is a kind of enclosure).
'ru'+'sty'='RUSTY'
'in' acts as a link.
(Other definitions for rusty that I've seen before include "Reddish," , "Impaired through lack of use" , "showing sign of inactivity?" , "Behind the times" , "a bit faded" .)