The most common and most likely answer to this crossword clue is the 9 letter word ANNULLING
I've seen this crossword clue in The Guardian
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Buddy explains!
The straight part
We know that "Cancellation" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cancellation = annulling.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does ANNULLING mean?
ANNULLING verb- To formally revoke the validity of.
- To dissolve (a marital union) on the grounds that it is not valid.
- An annulment.