Staid, solemn (5)
I believe the answer is:
sober
This clue is a double definition.
'staid' is the first definition.
(synonyms)
'solemn' is the second definition.
(synonyms)
(Other definitions for sober that I've seen before include "Temperate" , "Serious, dull" , "Earnest" , "Dignified and serious" , "Sedate and rational" .)