The most common and most likely answer to this crossword clue is the 11 letter word VICISSITUDE
I've seen this crossword clue in The Daily Telegraph
V
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C
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S
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Buddy explains!

The straight part
We know that "change of fortune" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case change of fortune = vicissitude.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 VICISSITUDE mean?
VICISSITUDE noun- Regular change or succession from one thing to another, or one part of a cycle to the next; alternation; mutual succession; interchange.
- (often in the plural) A change, especially in one's life or fortunes.