The most common and most likely answer to this crossword clue is the 8 letter word OVERTURN
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More clues leading to the result OVERTURN
Buddy explains!

The straight part
We know that "upset" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case upset = overturn.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 OVERTURN mean?
OVERTURN noun- A turning over or upside-down; inversion.
- The overturning or overthrow of some institution or state of affairs; ruin.
- To turn over, capsize or upset.
- To overthrow or destroy.
- To reverse (a decision); to overrule or rescind.
- To diminish the significance of a previous defeat by winning; to make a comeback from.