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