The most common and most likely answer to this crossword clue is the 6 letter word RUBRIC
I've seen this crossword clue in The Daily Telegraph
R
U
B
R
I
C
More clues leading to the result RUBRIC
- Instructions to chafe with a lot of wealthy
- British in charge follow sport’s guiding rule
- Regret loyal friend announced instructions to candidates
- Instructions irritate wealthy husband given boot
- Instructions irritate
- Problem over short outrageous heading
- Explanation of game with brother in charge
- Polish and almost wealthy, getting a distinguished title
- Instructions to massage deep, not hard
- Guide offering massage with first of rises in charge
Buddy explains!

The straight part
We know that "set of rules" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case set of rules = rubric.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 RUBRIC mean?
RUBRIC 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.
- To adorn with red; to redden.
- Coloured or marked with red; placed in rubrics.
- Of or relating to the rubric or rubrics; rubrical.