Horizontally level (4)
I believe the answer is:
flat
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">horizontally level</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for flat that I've seen before include "No longer charged" , "Dull; set of rooms" , "Not sloping; apartment" , "place to live" , "Apartment; plane" .)