Fair game? (3)
I believe the answer is:
fun
This could be a double definition.
'fair' is the first definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'game?' is the second definition.
(thesaurus)
Can you help me to learn more?
(Other definitions for fun that I've seen before include "Jollification" , "Light-hearted pleasure" , "Entertaining" , "Enjoyment" , "Playful amusement" .)