Level, not bumpy (4)
I believe the answer is:
even
I believe this is a double definition.
'level' is the first definition.
(I know that level can be written as even)
'not bumpy' is the second definition.
(I've seen this before)
(Other definitions for even that I've seen before include "Equable" , "Level, equal" , "Level, par" , "Stable" , "Unvarying" .)