The most common and most likely answer to this crossword clue is the 12 letter word BESPATTERING
I've seen this crossword clue in Financial Times
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More clues leading to the result BESPATTERING
Buddy explains!

The straight part
We know that "Defamation" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case defamation = bespattering.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 BESPATTERING mean?
BESPATTERING verb- To spatter or cover with something; sprinkle with anything liquid, or with any wet or adhesive substance.
- To soil by spattering.
- To asperse with calumny or reproach; shend.