Cryptic crossword clue
A law on mummification finally accepted by Roman ruler, not head of ancient Egypt 11

The most common and most likely answer to this crossword clue is the 11 letter word ALEXANDRIAN

I've seen this crossword clue in The Times
A
L
E
X
A
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Buddy explains!

The straight part
We know that "of ancient Egypt" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case of ancient egypt = alexandrian.
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.

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