Quits the flat (4)
I believe the answer is:
even
I believe this clue is a double definition.
'quits' is the first definition.
(as in 'call it quits')
'flat' is the second definition.
(level)
'the' acts as a link.
(Other definitions for even that I've seen before include "neck and neck" , "Level, straight or regular" , "Flat, level" , "Short joke" , "exact" .)