The most common and most likely answer to this crossword clue is the 9 letter word ECHINACEA
I've seen this crossword clue in The Times
E
C
H
I
N
A
C
E
A
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The straight part
We know that "Plant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case plant = echinacea.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 ECHINACEA mean?
ECHINACEA noun- Any of several plants, of genus Echinacea, having pinkish-purple flowers.
- A herbal medicine extracted from the roots and seeds of such plants.