The most common and most likely answer to this crossword clue is the 8 letter word RESCRIPT
I've seen this crossword clue in The Guardian
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Buddy explains!

The straight part
We know that "Decree" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case decree = rescript.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 RESCRIPT mean?
RESCRIPT noun- A clarification of a point of law by a monarch issued upon formal consultation by a lower magistrate.
- (canon law) An ad hoc reply of a pope to some specific question of canon law or morality, without precedential force, sometimes (improper) inclusive of decretals which serve as precedents in canon law.
- A duplicate copy of a legal document.
- A rewriting, a document copied or written again.
- To script again or anew.