Constant flush (4)
I believe the answer is:
even
I believe this clue is a double definition.
'constant' is the first definition.
(thesaurus)
'flush' is the second definition.
('even' can be similar in meaning to 'flush')
(Other definitions for even that I've seen before include "Late in the day" , "Iron - uniform - flat" , "like the numbers on two sides here" , "Level, equal" , "When darkness starts to fall" .)