Fair game? Not quite (4)
I believe the answer is:
even
'fair' is the definition.
(thesaurus)
'game? not quite' is the wordplay.
'game?' becomes 'event' (game is a kind of event).
'not quite' means to remove the last letter.
'event' with its final letter taken away is 'EVEN'.
(Other definitions for even that I've seen before include "Unvarying" , "Level, par" , "Going so far as to" , "time of day" , "end of the day, reading poetry" .)