As a return game, it's set up outside the pub (6)
I believe the answer is:
tennis
'pub' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'as a return game it's set up outside the' is the wordplay.
I cannot really understand how this works, but
'up' could be 'is' (I've seen this in other clues) and 'is' is present in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for tennis that I've seen before include "Sport on a court" , "Medieval, originally French, sport, codified by the British" , "Wimbledon sport" , "Pete Sampras's game" , "Sport with grand slams?" .)