The most common and most likely answer to this crossword clue is the 7 letter word RUBRICS
I've seen this crossword clue in Financial Times
R
U
B
R
I
C
S
Buddy explains!

The straight part
We know that "Leading rules" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case leading rules = rubrics.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 RUBRICS mean?
RUBRICS noun- A heading in a book highlighted in red.
- A title of a category or a class.
- The directions for a religious service, formerly printed in red letters.
- An established rule or custom; a guideline.
- A printed set of scoring criteria for evaluating student work and for giving feedback.
- A flourish after a signature.
- Red ochre.