Quite flat (4)
I believe the answer is:
even
This clue is a double definition.
'quite' is the first definition.
(thesaurus)
'flat' is the second definition.
(level)
(Other definitions for even that I've seen before include "Flat - yet" , "Still - flat" , "Flush" , "Level, straight or regular" , "When darkness starts to fall" .)