Sombre, grave (6)
I believe the answer is:
solemn
This clue is a double definition.
'sombre' is the first definition.
(synonyms)
'grave' is the second definition.
(synonyms)
(Other definitions for solemn that I've seen before include "Formal, ceremonial" , "'Earnest, grave (6)'" , "Marked by pomp or ceremony" , "Formal, dignified" , "Melons (anag.)" .)