The most common and most likely answer to this crossword clue is the 9 letter word GLADIOLUS
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Buddy explains!

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 = gladiolus.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 GLADIOLUS mean?
GLADIOLUS noun- The center part of the sternum.
- Any of several flowering plants, of the genus Gladiolus, having sword-shaped leaves and showy flowers on spikes; gladiola.