The most common and most likely answer to this crossword clue is the 8 letter word SKULLCAP
S
K
U
L
L
C
A
P
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 = skullcap.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 SKULLCAP mean?
SKULLCAP noun- A small domed cap that covers the area from the forehead to just above the back of the neck.
- A yarmulke-like hat worn as an element of ghetto fashion.
- The calvaria, the top part of the skull, covering the cranial cavity containing the brain.
- Any of several species of flowering plants of the genus Scutellaria, in the Lamiaceae family.
- A torture device for compressing the skull.