Cryptic crossword clue
You and Stephen changed side 10

The most common and most likely answer to this crossword clue is the 10 letter word HYPOTENUSE

I've seen this crossword clue in Financial Times
H
Y
P
O
T
E
N
U
S
E

Buddy explains!

The straight part
We know that "side" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case side = hypotenuse.
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 HYPOTENUSE mean?

HYPOTENUSE noun
  1. The side of a right triangle opposite the right angle.

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