Flat and smooth (4)
I believe the answer is:
even
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">flat and smooth</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for even that I've seen before include "Flat, level" , "without a rise?" , "notwithstanding" , "Short joke" , "Still - divisible by two" .)