The most common and most likely answer to this crossword clue is the 6 letter word LACHES
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The straight part
We know that "Legal negligence" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case legal negligence = laches.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 LACHES mean?
LACHES noun- An unreasonable delay in bringing a claim alleging a wrong, which means the person who waited shall not be permitted to seek an equitable remedy because the delay prejudiced the moving party. Sleeping on one's rights.