An upsetting accident (5)
I believe the answer is:
spill
'accident' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for spill that I've seen before include "Fire lighter; reveal (confidential information)" , "Shed" , "Oil tanker mishap?" , "accident" , "Waste from container" .)