Peak or zenith (4)
I believe the answer is:
apex
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">peak</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that peak can be written as apex)</span><br/ ><br/ >'<span class="explanation-format__original">zenith</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 apex that I've seen before include "Pointed top" , "Apogee, summit" , "Culmination" , "Top point" , "Summit or peak" .)