Eternal sin, playing old game (4,6)
I believe the answer is:
real tennis
'old game' is the definition.
'real tennis' can be an answer for 'game' (real tennis is a kind of game). I'm unsure of the 'old' bit.
'eternal sin playing' is the wordplay.
'playing' indicates anagramming the letters.
'eternal'+'sin'='eternalsin'
'eternalsin' anagrammed gives 'REAL TENNIS'.
Can you help me to learn more?
(Other definitions for real tennis that I've seen before include "Old sport" , "ancient game" , "In which court member served" , "Game played in court" , "Early form of Wimbledon's game" .)