The most common and most likely answer to this crossword clue is the 4 plus 5 letter word LONG REIGN
I've seen this crossword clue in The Guardian
L
O
N
G
R
E
I
G
N
Buddy explains!
The straight part
We know that "extended period at the top" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case extended period at the top = long reign.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.