The most common and most likely answer to this crossword clue is the 3 plus 4 plus 3 plus 5 letter word GOD SAVE THE QUEEN
G
O
D
S
A
V
E
T
H
E
Q
U
E
E
N
Buddy explains!
The straight part
We know that "call for ruler’s divine protection" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case call for ruler’s divine protection = god save the queen.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.