The most common and most likely answer to this crossword clue is the 9 letter word APPELLANT
I've seen this crossword clue in The Times
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More clues leading to the result APPELLANT
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The straight part
We know that "one making plea" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one making plea = appellant.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 APPELLANT mean?
APPELLANT noun- A litigant or party that is making an appeal in court
- One who makes an earnest entreaty of any kind.
- One who challenges another to single combat.
- One of the clergy in the Jansenist controversy who rejected the bull Unigenitus issued in 1713, appealing to a pope "better informed", or to a general council.
- Of or relating to appeals; appellate
- In the process of appealing