The most common and most likely answer to this crossword clue is the 10 letter word DISCUSSANT
I've seen this crossword clue in The Times
D
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Buddy explains!

The straight part
We know that "debater" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case debater = discussant.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 DISCUSSANT mean?
DISCUSSANT noun- Someone involved in a discussion, especially a participant in a formal discussion or who has been assigned a particular role or topic.