Quits flat (4)
I believe the answer is:
even
I believe this is a double definition.
'quits' is the first definition.
(as in 'call it quits')
'flat' is the second definition.
('even' can be similar in meaning to 'flat')
(Other definitions for even that I've seen before include "Like 2018 but not 2017" , "Like half the numbers" , "Level, calm" , "Not easily annoyed" , "exact" .)