Grave, sombre (6)
I believe the answer is:
solemn
I believe this is a double definition.
'grave' is the first definition.
(I know that grave can be written as solemn)
'sombre' is the second definition.
(synonyms)
(Other definitions for solemn that I've seen before include "Formal, dignified" , "'Earnest, grave (6)'" , "'Dignified, sombre (6)'" , "Formal, ceremonius" , "Serious and humourless" .)