The most common and most likely answer to this crossword clue is the 6 plus 6 letter word SECOND NATURE
I've seen this crossword clue in The Daily Telegraph
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More clues leading to the result SECOND NATURE
- Innate tendency to censure and abuse
- Mistakenly denounces art that’s now done without thinking
- Support life in the wild — it’s instinctive
- Criminal case turned on deeply ingrained habit
- It’s one’s characteristic habit to back the Constitution?
- Revolutionary denounces art: it’s almost instinct
- Instant, kind behaviour that's become habitual
Buddy explains!
The straight part
We know that "done without thinking" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case done without thinking = second nature.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.