The most common and most likely answer to this crossword clue is the 7 letter word MAHONIA
I've seen this crossword clue in The Times
M
A
H
O
N
I
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 = mahonia.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 MAHONIA mean?
MAHONIA noun- Any of the genus Mahonia (now often included in Berberis) of evergreen shrubs.