The most common and most likely answer to this crossword clue is the 6 letter word DEFAME
I've seen this crossword clue in The Guardian
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Buddy explains!

The straight part
We know that "Slander" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case slander = defame.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 DEFAME mean?
DEFAME noun- Disgrace, dishonour.
- Defamation; slander, libel.
- To disgrace; to bring into disrepute.
- To charge; to accuse (someone) of an offence.
- To harm or diminish the reputation of; to disparage.