Rock bottom (5)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">rock bottom</span>' is the definition.<br/ ><span class="explanation-format__explanation">(a nadir is something's lowest point)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for nadir that I've seen before include "Point of greatest despair" , "deep despair" , "Low spot" , "All-time low" , "Rock bottom" .)