The most common and most likely answer to this crossword clue is the 9 letter word CENTAUREA
I've seen this crossword clue in The Guardian
C
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A
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The straight part
We know that "star thistle, for example" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case star thistle, for example = centaurea.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 CENTAUREA mean?
CENTAUREA noun- Any of the flowering plants in or formerly in the genus Centaurium.
- Any of diverse other plants: