Sad or dull (6)
I believe the answer is:
dreary
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">sad</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >'<span class="explanation-format__original">dull</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ ></span>
(Other definitions for dreary that I've seen before include "Dismal, depressing" , "Dismal, funeral" , "Grey and dull" , "Dismal, boring" , "Cheerless and depressing" .)