The most common and most likely answer to this crossword clue is the 9 letter word ATAVISTIC
I've seen this crossword clue in Financial Times
A
T
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V
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Buddy explains!
The straight part
We know that "reverting to old behaviour" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case reverting to old behaviour = atavistic.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 ATAVISTIC mean?
ATAVISTIC adjective- Of the recurrence of a trait reappearing after an absence of one or more generations due to a chance recombination of genes.
- Of a throwback or exhibiting primitivism.
- Relating to earlier, more primitive behavior that returns after an absence.