The most common and most likely answer to this crossword clue is the 12 letter word PREPONDERATE
I've seen this crossword clue in The Daily Telegraph
P
R
E
P
O
N
D
E
R
A
T
E
More clues leading to the result PREPONDERATE
Buddy explains!

The straight part
We know that "rule" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rule = preponderate.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 PREPONDERATE mean?
PREPONDERATE verb- To outweigh; to be heavier than; to exceed in weight
- To overpower by stronger or moral power.
- To cause to prefer; to incline; to decide.
- To exceed in weight; hence, to predominate