On reflection, offender admitting dubious tale is game in court (4,6)
I believe the answer is:
real tennis
'game in court' is the definition.
'real tennis' can be an answer for 'game' (real tennis is a kind of game). I am not sure about the 'in court' bit.
'on reflection offender admitting dubious tale' is the wordplay.
'on reflection' says the letters should be written backwards.
'offender' becomes 'sinner' (sinner is a kind of offender).
'admitting' means one lot of letters goes inside another (admit can mean to welcome in or accept).
'dubious' indicates anagramming the letters.
'sinner' reversed gives 'rennis'.
'tale' anagrammed gives 'ealt'.
'rennis' going around 'ealt' is 'REAL TENNIS'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for real tennis that I've seen before include "Early form of Wimbledon's game" , "Old sport" , "Game played in court" , "ancient game" , "In which court member served" .)