Zenith, peak (4)
I believe the answer is:
acme
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">zenith</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">peak</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ ></span>
(Other definitions for acme that I've seen before include "crown" , "Summit of achievement" , "Height of excellence" , "highest position" , "crisis in history" .)