Serious, grave (6)
I believe the answer is:
solemn
I believe this clue is a double definition.
'serious' is the first definition.
(similar in meaning)
'grave' is the second definition.
(synonyms)
(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" .)