Vertex (4)
I believe the answer is:
apex
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">vertex</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that vertex can be written as apex)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for apex that I've seen before include "Point or summit" , "Peak, vertex" , "Summit - pinnacle" , "Absolute peak" , "reasonable fare" .)