Flat race possibly unfinished (4)
I believe the answer is:
even
'flat' is the definition.
('even' can be similar in meaning to 'flat')
'race possibly unfinished' is the wordplay.
'race possibly' becomes 'event' (race is a kind of event).
'unfinished' 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 "time of day" , "Placid - not easily angered" , "Equable" , "Not odd; flat" , "Yet; smooth" .)