Cruel fly he swatted with glee (10)

I believe the answer is:

cheerfully

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

'swatted with glee' is the definition.
I can't judge whether this definition defines the answer.

'cruel fly he' is the wordplay.
'cruel fly he' can be anagrammed to 'CHEERFULLY'.
But, I am not clear how this is indicated.

Can you help me to learn more?

(Other definitions for cheerfully that I've seen before include "In a happy manner" , "In happy fashion" .)

I've seen this clue in the Sydney Morning Herald.

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