Jolly, are you joking? (9)

I believe the answer is:

seriously

I'm a little stuck... Click here to teach me more about this clue!

'jolly' is the definition.
Both the answer and definition are adverbs. Perhaps you can see an association between them that I don't see?

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for seriously that I've seen before include "In a solemn and thoughtful manner" , "Gravely" .)

I've seen this clue in The Telegraph.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.