Then sir GIO managed speculation (10)

I believe the answer is:

theorising

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'then sir gio managed speculation' is the definition.
I can't judge whether this definition defines the answer.

'then sir gio managed' is the wordplay.
'managed' indicates an anagram.
'then'+'sir'+'gio'='thensirgio'
'thensirgio' with letters rearranged gives 'THEORISING'.

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(Other definitions for theorising that I've seen before include "indulging in speculation" , "coming up with ideas?" , "In speculative activity" .)

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

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