Distance downwards (5)
I believe the answer is:
depth
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">distance downwards</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for depth that I've seen before include "Measurement from front to back" , "Distance from top down as in water" , "Intensity (of emotion)" , "Insight" , "Thoroughness" .)