The most common and most likely answer to this crossword clue is the 7 letter word DEBATER
I've seen this crossword clue in The Guardian
D
E
B
A
T
E
R
More clues leading to the result DEBATER
Buddy explains!

The straight part
We know that "One discussing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one discussing = debater.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 DEBATER mean?
DEBATER noun- One who debates or participates in a debate; one who argues.