The most common and most likely answer to this crossword clue is the 12 letter word JURISDICTION
I've seen this crossword clue in The Times
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Buddy explains!

The straight part
We know that "Legal power?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case legal power? = jurisdiction.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 JURISDICTION mean?
JURISDICTION noun- The power, right, or authority to interpret and apply the law.
- The power or right to exercise authority.
- The power or right to perform some action as part of applying the law.
- The authority of a sovereign power to govern or legislate.
- The limits or territory within which authority may be exercised.