Jolly, in a solemn way! (9)
I believe the answer is:
seriously
This clue might be a double definition.
'jolly' is the first definition.
Both the definition and answer are adverbs. Perhaps you can see a link between them that I don't see?
'in a solemn way' is the second definition.
(I know this)
Can you help me to learn more?
(Other definitions for seriously that I've seen before include "In a solemn and thoughtful manner" , "Gravely" .)