The most common and most likely answer to this crossword clue is the 8 letter word ROGATION
R
O
G
A
T
I
O
N
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The straight part
We know that "petition" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case petition = rogation.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 ROGATION mean?
ROGATION noun- A deeply serious and somber prayer or entreaty.
- The demand, by the consuls or tribunes, of a law to be passed by the people; a proposed law or decree.