The most common and most likely answer to this crossword clue is the 12 letter word RESTRICTIONS
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More clues leading to the result RESTRICTIONS
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 = restrictions.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 RESTRICTIONS mean?
RESTRICTIONS noun- The act of restricting, or the state of being restricted.
- A regulation or limitation that restricts.
- The mechanism by which a cell degrades foreign DNA material.