The most common and most likely answer to this crossword clue is the 7 letter word DOGBANE
I've seen this crossword clue in Financial Times
D
O
G
B
A
N
E
More clues leading to the result DOGBANE
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 = dogbane.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 DOGBANE mean?
DOGBANE noun- Any species of genus Apocynum, eponymous of the dogbane family Apocynaceae