The most common and most likely answer to this crossword clue is the 9 letter word CINERARIA
I've seen this crossword clue in The Daily Telegraph
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More clues leading to the result CINERARIA
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 = cineraria.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 CINERARIA mean?
CINERARIA noun- A place or receptacle for depositing the ashes of cremated people.
- Any of the genus Cineraria of flowering plants in the sunflower family.
- Any of the garden flowers in the species Pericallis × hybrida (formerly classified in the genus Cineraria)