The most common and most likely answer to this crossword clue is the 8 letter word VALERIAN
I've seen this crossword clue in Independent
V
A
L
E
R
I
A
N
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 = valerian.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 VALERIAN mean?
VALERIAN noun- A hardy perennial flowering plant, Valeriana officinalis, with heads of sweetly scented pink or white flowers.
- More generally, any plant of the genus Valeriana.
- The root of Valeriana officinalis, used in herbal medicine.
- An extract of the dried roots of the Valeriana officinalis used in herbal medicine as a sedative.