The most common and most likely answer to this crossword clue is the 13 letter word HEART‑RENDING
I've seen this crossword clue in The Guardian
H
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Buddy explains!
The straight part
We know that "very upsetting" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case very upsetting = heart‑rending.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 HEART‑RENDING mean?
HEART‑RENDING adjective- That causes great grief, anguish or distress.
- That elicits deep sympathy.