The most common and most likely answer to this crossword clue is the 6 letter word AXIOMS
I've seen this crossword clue in The Guardian
A
X
I
O
M
S
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 = axioms.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 AXIOMS mean?
AXIOMS noun- A seemingly self-evident or necessary truth which is based on assumption; a principle or proposition which cannot actually be proved or disproved.
- (proof theory) A fundamental assumption that serves as a basis for deduction of theorems; a postulate (sometimes distinguished from postulates as being universally applicable, whereas postulates are particular to a certain science or context).
- An established principle in some artistic practice or science that is universally received.