The most common and most likely answer to this crossword clue is the 10 letter word ALGORITHMS
I've seen this crossword clue in The Guardian
A
L
G
O
R
I
T
H
M
S
More clues leading to the result ALGORITHMS
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 = algorithms.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 ALGORITHMS mean?
ALGORITHMS noun- A collection of ordered steps that solve a mathematical problem. A precise step-by-step plan for a computational procedure that possibly begins with an input value and yields an output value in a finite number of steps.
- Calculation with Arabic numerals; algorism.