The most common and most likely answer to this crossword clue is the 5 plus 8 letter word COURT CIRCULAR
I've seen this crossword clue in The Daily Telegraph
C
O
U
R
T
C
I
R
C
U
L
A
R
Buddy explains!
The straight part
We know that "report of royals" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case report of royals = court circular.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.