The most common and most likely answer to this crossword clue is the 9 letter word INSULTING
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More clues leading to the result INSULTING
Buddy explains!

The straight part
We know that "defamatory" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case defamatory = insulting.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 INSULTING mean?
INSULTING verb- To be insensitive, insolent, or rude to (somebody); to affront or demean (someone).
- To assail, assault, or attack; (specifically) to carry out an assault, attack, or onset without preparation.
- To behave in an obnoxious and superior manner (against or over someone).
- To leap or trample upon.
- The act of giving insult.
- Containing insult, or having the intention of insulting.