The most common and most likely answer to this crossword clue is the 13 letter word FREE-STANDING
I've seen this crossword clue in The Guardian
F
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Buddy explains!
The straight part
We know that "autonomous" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case autonomous = free-standing.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 FREE-STANDING mean?
FREE-STANDING adjective- Not fixed in any way to a surface. Relying solely on the force of gravity to stay in place
- That stands alone, not supported at either side
- Independent, autonomous