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.
(I know that grave can be written as solemn)
(Other definitions for solemn that I've seen before include "Formal, ceremonial" , "'Earnest, grave (6)'" , "Marked by pomp or ceremony" , "Formal, dignified" , "Melons (anag.)" .)