The most common and most likely answer to this crossword clue is the 10 letter word PRINCIPLES
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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 = principles.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 PRINCIPLES mean?
PRINCIPLES noun- A fundamental assumption or guiding belief.
- A rule used to choose among solutions to a problem.
- (sometimes pluralized) Moral rule or aspect.
- A rule or law of nature, or the basic idea on how the laws of nature are applied.
- A fundamental essence, particularly one producing a given quality.
- A beginning.
- A source, or origin; that from which anything proceeds; fundamental substance or energy; primordial substance; ultimate element, or cause.
- An original faculty or endowment.
- To equip with principles; to establish, or fix, in certain principles; to impress with any tenet or rule of conduct.