Happy chappies wiping backsides for fun (4)
I believe the answer is:
game
'fun' is the definition.
(thesaurus)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for game that I've seen before include "eg Badminton" , "See 3" , "Somehow disabled" , "Hunted wild animals" , "For instance, go" .)