Regular flat (4)
I believe the answer is:
even
This clue is a double definition.
'regular' is the first definition.
(I know that regular can be written as even)
'flat' is the second definition.
(level)
(Other definitions for even that I've seen before include "Yet; smooth" , "Equally balanced; level" , "neck and neck" , "time of day" , "notwithstanding" .)