Regular; flush (4)
I believe the answer is:
even
I believe this clue is a double definition.
'regular' is the first definition.
(I know that regular can be written as even)
'flush' is the second definition.
(I know that flush can be written as even)
(Other definitions for even that I've seen before include "Like 2018 but not 2017" , "no-win situation?" , "All square" , "Flat, level" , "Yet; smooth" .)