The most common and most likely answer to this crossword clue is the 7 plus 3 plus 4 letter word WILLIAM AND MARY
I've seen this crossword clue in The Times
W
I
L
L
I
A
M
A
N
D
M
A
R
Y
Buddy explains!
The straight part
We know that "They ruled" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they ruled = william and mary.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.