Murk (5)
I believe the answer is:
gloom
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">murk</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 gloom that I've seen before include "Melancholy state" , "Darkness; depression" , "Dimness" , "Murkiness" , "Obscurity or melancholy" .)