How sick one is of having to be exuberant? (8)

I believe the answer is:

heartily

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

'how' is the definition.
Both the definition and answer are adverbs. Maybe you can see a link between them that I don't see?

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for heartily that I've seen before include "Enthusiastically" , "To a great degree" , "With gusto and without reservation" , "thoroughly" , "With vigour" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app