The most common and most likely answer to this crossword clue is the 6 letter word OSWALD
I've seen this crossword clue in The Guardian
O
S
W
A
L
D
More clues leading to the result OSWALD
- Political assassin having died, rule thus lifted
- Fascist leader’s senile hugging was creepy
- King once setting up some stupid laws ostentatiously
- Was shot, captured by familiar killer
- Killer revolutionary breaking party rules
- Old king’s assassin
- Assassin? Oliver Stone wouldn’t accept legal decision initially
- Old king reversed statutes when pressed by old duke
- Drunk was collared by elderly assassin
- Saintly northern ruler reversing some stupid laws, obviously
Buddy explains!

The straight part
We know that "Saintly northern ruler" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case saintly northern ruler = oswald.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.