The most common and most likely answer to this crossword clue is the 11 letter word REGULATIONS
I've seen this crossword clue in The Guardian
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More clues leading to the result REGULATIONS
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 = regulations.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 REGULATIONS mean?
REGULATIONS noun- The act of regulating or the condition of being regulated.
- A law or administrative rule, issued by an organization, used to guide or prescribe the conduct of members of that organization.
- A type of law made by the executive branch of government, usually by virtue of a statute made by the legislative branch giving the executive the authority to do so.
- (European Union law) A form of legislative act which is self-effecting, and requires no further intervention by the Member States to become law.
- Mechanism controlling DNA transcription.
- Physiological process which consists in maintaining homoeostasis.