Sombre (6)
'sombre' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for solemn that I've seen before include "Grave and serious about melons" , "Portentous" , "'Sedate, grave (6)'" , "Melons (anag.)" , "gloomy" .)
(Other definitions for morose that I've seen before include "Melancholy" , "Sullen or glum." , "Lugubrious" , "Down in the dumps" , "Peevish and gloomy" .)