... you can annul, finally, formally signing and separating (9)

I believe the answer is:

unlinking

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'signing and separating' is the definition.
I can't tell whether this definition defines the answer.

'you can annul finally formally' is the wordplay.
'finally' says to take the final letters.
'formally' becomes 'inking' (I can't justify this - if you can you should believe this answer much more).
The final letters of 'you can annul' are 'unl'.
'unl'+'inking'='UNLINKING'

Can you help me to learn more?

I've seen this clue in The Hindu.

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