Still flat (4)
I believe the answer is:
even
I believe this clue is a double definition.
'still' is the first definition.
('even' can be a synonym of 'still')
'flat' is the second definition.
('even' can be similar in meaning to 'flat')
(Other definitions for even that I've seen before include "Level, calm" , "Nearly" , "All square" , "Pantomime" , "notwithstanding" .)