Vertical, perpendicular (5)
I believe the answer is:
plumb
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">vertical</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ >'<span class="explanation-format__original">perpendicular</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ ></span>
(Other definitions for plumb that I've seen before include "Absolutely straight - sound" , "Lead weight; exactly" , "Check depth of water below" , "Fathom - out-and-out" , "Going straight up and down" .)