The most common and most likely answer to this crossword clue is the 8 letter word MANDAMUS
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The straight part
We know that "Legal order" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case legal order = mandamus.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 MANDAMUS mean?
MANDAMUS noun- A common law prerogative writ that compels a court or government officer to perform mandatory or purely ministerial duties correctly.
- To serve a writ of this kind upon.