Cryptic crossword clue
Characters changing used to be random vowels

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

I've seen this crossword clue in Independent
W
E
R
E
W
O
L
V
E
S

Buddy explains!

The straight part
We know that "Characters changing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case characters changing = werewolves.
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 WEREWOLVES mean?

WEREWOLVES noun
  1. A person who is transformed or can transform into a wolf or a wolflike human, often said to transform during a full moon.

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