Seriously fractured arm in silly venture (8)

I believe the answer is:

formally

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'seriously' is the definition.
(thesaurus)

'fractured arm in silly venture' is the wordplay.
'fractured' is an anagram indicator.
'in' indicates putting letters inside.
'silly venture' becomes 'folly' (I can't justify this - if you can you should believe this answer much more).
'arm' is an anagram of 'rma'.
'rma' inserted inside 'folly' is 'FORMALLY'.

Can you help me to learn more?

(Other definitions for formally that I've seen before include "following conventional rules" , "With official authorisation" , "in precise terms" , "According to the rules" , "without ceremony" .)

I've seen this clue in The Irish Times.

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