The most common and most likely answer to this crossword clue is the 10 letter word OVERTHROWN
I've seen this crossword clue in The Guardian
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Buddy explains!

The straight part
We know that "ousted" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ousted = overthrown.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 OVERTHROWN mean?
OVERTHROWN verb- To bring about the downfall of (a government, etc.), especially by force.
- To throw down to the ground, to overturn.
- To throw (something) so that it goes too far.