Low-water mark (5)
I believe the answer is:
nadir
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">low-water mark</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that low-water mark can be written as nadir)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for nadir that I've seen before include "businessman formerly escaping justice" , "Point opposite the zenith" , "deep despair" , "Dinar (anag.)" , "Floor of drain in need of repair" .)