Not odd; flat and smooth (4)
I believe the answer is:
even
I believe this clue is a double definition.
'not odd' is the first definition.
('even' is the opposite to 'odd')
'flat and smooth' is the second definition.
(I've seen this before)
(Other definitions for even that I've seen before include "of consistent quality" , "Flat - yet" , "Regular - smooth" , "Level as odds may be" , "Tied" .)