The most common and most likely answer to this crossword clue is the 5 letter word SPILL
I've seen this crossword clue in Financial Times
S
P
I
L
L
More clues leading to the result SPILL
- Small tablet found in shed
- Small contraceptive leak
- Bridge player has medication causing upset
- Fall sick after swamp’s drained
- Shed yet to receive power for tool’s lead
- Shed light
- Loss of oil perhaps from lighter
- Medicine needed after son’s fall from horse
- Accident to make light of?
- Not quite stupid to allow parking in shed
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 = spill.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 SPILL mean?
SPILL noun- A mess of something that has been dropped.
- A fall or stumble.
- A small stick or piece of paper used to light a candle, cigarette etc by the transfer of a flame from a fire.
- A slender piece of anything.
- One of the thick laths or poles driven horizontally ahead of the main timbering in advancing a level in loose ground.
- (sound recording) The situation where sound is picked up by a microphone from a source other than that which is intended.
- A small sum of money.
- (Australian politics) A declaration that the leadership of a parliamentary party is vacant, and open for re-election. Short form of leadership spill.
- To drop something so that it spreads out or makes a mess; to accidentally pour.
- To spread out or fall out, as above.
- To drop something that was intended to be caught.
- To mar; to damage; to destroy by misuse; to waste.
- To be destroyed, ruined, or wasted; to come to ruin; to perish; to waste.
- To cause to flow out and be lost or wasted; to shed.
- To cover or decorate with slender pieces of wood, metal, ivory, etc.; to inlay.
- To relieve a sail from the pressure of the wind, so that it can be more easily reefed or furled, or to lessen the strain.
- (Australian politics) To open the leadership of a parliamentary party for re-election.
- To reveal information to an uninformed party.
- (of a knot). To come undone.