Flat — level (5)
I believe the answer is:
plane
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">flat</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that flat can be written as plane)</span><br/ ><br/ >'<span class="explanation-format__original">level</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that plane is a type of level)</span><br/ ><br/ ></span>
(Other definitions for plane that I've seen before include "Smoother" , "Tree - figure - aircraft" , "Smooth" , "Form of transport" , "Nepal (anag.)" .)