Culmination, climax (6)
I believe the answer is:
apogee
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">culmination</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >'<span class="explanation-format__original">climax</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 apogee that I've seen before include "Highest point (of orbit)" , "Culmination, climax" , "crown" , "Point in the orbit of the moon that's furthest from the earth" , "Highest or most distant point, or climax" .)