The most common and most likely answer to this crossword clue is the 8 letter word MILKWEED
I've seen this crossword clue in The Daily Telegraph
M
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L
K
W
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D
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 = milkweed.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 MILKWEED mean?
MILKWEED noun- Any of several plants that have a milky sap and have pods that split to release seeds with silky tufts.
- A monarch butterfly (Danaus spp).