The most common and most likely answer to this crossword clue is the 10 letter word CHAMELEONS
I've seen this crossword clue in The Daily Telegraph
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Buddy explains!
The straight part
We know that "Changed circumstances may put a different complexion on them" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case changed circumstances may put a different complexion on them = chameleons.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 CHAMELEONS mean?
CHAMELEONS noun- A small to mid-size reptile, of the family Chamaeleonidae, and one of the best known lizard families able to change color and project its long tongue.
- A person with inconstant behavior; one able to quickly adjust to new circumstances.
- A hypothetical scalar particle with a non-linear self-interaction, giving it an effective mass that depends on its environment: the presence of other fields.