Serious, grave (6)
I believe the answer is:
solemn
<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">(similar in meaning)</span><br/ ><br/ >'<span class="explanation-format__original">grave</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ ></span>
(Other definitions for solemn that I've seen before include "pious" , "Serious, dignified or ceremonial" , "'Sedate, grave (6)'" , "Marked by pomp or ceremony" , "Sober and serious" .)