The most common and most likely answer to this crossword clue is the 13 letter word RESTRUCTURING
I've seen this crossword clue in Financial Times
R
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Buddy explains!
The straight part
We know that "Shake-up/3" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case shake-up/3 = restructuring.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 RESTRUCTURING mean?
RESTRUCTURING verb- To change the organization of.
- To modify the terms of a loan, providing relief to a debtor who would otherwise be forced to default.
- A reorganization; an alteration of structure.