Zenith's opposite (5)
I believe the answer is:
nadir
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">zenith's opposite</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for nadir that I've seen before include "Point opposite the zenith" , "Low spot" , "pits" , "worst-case scenario?" , "lowest of the low" .)