Serious, gloomy (4)
I believe the answer is:
grim
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">serious</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >'<span class="explanation-format__original">gloomy</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that gloomy can be written as grim)</span><br/ ><br/ ></span>
(Other definitions for grim that I've seen before include "Very serious/gloomy" , "looking black" , "Dour and severe" , "Harsh and merciless" , "Serious; dour" .)