The most common and most likely answer to this crossword clue is the 12 letter word CONSTITUTION
I've seen this crossword clue in The Daily Telegraph
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Buddy explains!
The straight part
We know that "Basic rules/health" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case basic rules/health = constitution.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 CONSTITUTION mean?
CONSTITUTION noun- The act, or process of setting something up, or establishing something; the composition or structure of such a thing; its makeup.
- The formal or informal system of primary principles and laws that regulates a government or other institutions.
- A legal document describing such a formal system.
- A person's physical makeup or temperament, especially in respect of robustness.
- The general health of a person.