The most common and most likely answer to this crossword clue is the 6 letter word SPITED
I've seen this crossword clue in Financial Times
S
P
I
T
E
D
More clues leading to the result SPITED
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 = spited.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 SPITED mean?
SPITED verb- To treat maliciously; to try to injure or thwart.
- To be angry at; to hate.
- To fill with spite; to offend; to vex.