Your flu is developing in a violent manner (9)

I believe the answer is:

furiously

'in a violent manner' is the definition.

'your flu is developing' is the wordplay.
'developing' is an anagram indicator.
'your'+'flu'+'is'='yourfluis'
'yourfluis' with letters rearranged gives 'FURIOUSLY'.

(Another definition for furiously that I've seen is " with violent passion".)

I've seen this clue in The Telegraph.

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