The most common and most likely answer to this crossword clue is the 9 letter word BACKSLIDE
I've seen this crossword clue in The Guardian
B
A
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More clues leading to the result BACKSLIDE
- Lose ground as learner crashes into rear end
- Stern about left showing return to old habits
- Relapse, injecting a bit of laudanum into bottom
- Second small fib about Democrat’s fall from grace
- Bottom line included relapse
- Defenders lied wickedly showing lapse in morals
- Left in rear, go astray
- Slip behind outside left
- Beginning of leak in rear sink again
Buddy explains!

The straight part
We know that "return to old habits" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case return to old habits = backslide.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 BACKSLIDE mean?
BACKSLIDE noun- A backward regression; a reverting back to a worse state.
- A dance move in which the feet are alternately slid back and the heels lifted, giving the illusion of walking forwards while actually moving backwards; later popularly called the moonwalk.
- To regress; to slip backwards or revert to a previous, worse state.
- To shirk responsibility; to renege on one's obligations or commitments.