The most common and most likely answer to this crossword clue is the 8 letter word CAPSIZED
I've seen this crossword clue in The Times
C
A
P
S
I
Z
E
D
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 = capsized.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 CAPSIZED mean?
CAPSIZED verb- To overturn.
- To cause (a ship) to overturn.
- (of knots) To deform under stress.