The most common and most likely answer to this crossword clue is the 6 letter word TUMULT
I've seen this crossword clue in Financial Times
T
U
M
U
L
T
Buddy explains!

The straight part
We know that "Upheaval" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case upheaval = tumult.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 TUMULT mean?
TUMULT noun- Confused, agitated noise as made by a crowd.
- Violent commotion or agitation, often with confusion of sounds.
- A riot or uprising.
- To make a tumult; to be in great commotion.