The most common and most likely answer to this crossword clue is the 12 letter word INADMISSIBLE
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Buddy explains!
The straight part
We know that "not valid in court" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not valid in court = inadmissible.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 INADMISSIBLE mean?
INADMISSIBLE noun- A person who is not to be admitted (to a country, a group, etc).
- Not admissible, especially that cannot be admitted as evidence at a trial