Flat, horizontal (5)
I believe the answer is:
level
<span class="explanation-format__ClueExplanation">I believe this clue 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 level)</span><br/ ><br/ >'<span class="explanation-format__original">horizontal</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ ></span>
(Other definitions for level that I've seen before include "Floor; horizontal" , "Horizontal plane - a palindrome" , "Neither winning, nor losing" , "Balanced" , "Device ot establish a flat surface" .)