Flat race isn't finished (4)
I believe the answer is:
even
'flat' is the definition.
(level)
'race isn't finished' is the wordplay.
'race' becomes 'event' (I've seen this before).
'isn't finished' means to remove the last letter.
'event' with its last letter taken away is 'EVEN'.
(Other definitions for even that I've seen before include "like six or twelve" , "Tied" , "Yet; smooth" , "Still - quits" , "Flush" .)