The most common and most likely answer to this crossword clue is the 7 letter word FLUSTER
F
L
U
S
T
E
R
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The straight part
We know that "Upset" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case upset = fluster.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 FLUSTER mean?
FLUSTER noun- A state of being flustered; overwrought confusion.
- To make hot and rosy, as with drinking.
- (by extension) To confuse; befuddle; throw into panic by making overwrought with confusion.
- To be in a heat or bustle; to be agitated and confused.