The most common and most likely answer to this crossword clue is the 8 letter word ADJUDGES
I've seen this crossword clue in The Guardian
A
D
J
U
D
G
E
S
More clues leading to the result ADJUDGES
Buddy explains!

The straight part
We know that "Rules" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rules = adjudges.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 ADJUDGES mean?
ADJUDGES verb- To declare to be.
- To deem or determine to be.
- To award judicially; to assign.