The most common and most likely answer to this crossword clue is the 10 letter word DISCOMPOSE
I've seen this crossword clue in Financial Times
D
I
S
C
O
M
P
O
S
E
More clues leading to the result DISCOMPOSE
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 = discompose.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 DISCOMPOSE mean?
DISCOMPOSE verb- To destroy the composure of; to disturb or agitate.
- To disarrange, or throw into a state of disorder.