Smooth uniform (4)
I believe the answer is:
even
I believe this is a double definition.
'smooth' is the first definition.
(I know that smooth can be written as even)
'uniform' is the second definition.
(both can mean equal or level)
(Other definitions for even that I've seen before include "Not bumpy" , "Deep crisp snow seen on Boxing Day also this" , "Regular; calm" , "All square" , "Not odd; flat" .)