The most common and most likely answer to this crossword clue is the 9 letter word DETHRONED
I've seen this crossword clue in The Daily Telegraph
D
E
T
H
R
O
N
E
D
Buddy explains!

The straight part
We know that "with authority usurped" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case with authority usurped = dethroned.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.
What does DETHRONED mean?
DETHRONED verb- To depose; to forcibly relieve a monarch of the monarchy.
- To remove any governing authority from power.
- To remove from any position of high status or power.
- To remove (something) from a position of power or paramount importance.