The most common and most likely answer to this crossword clue is the 8 letter word CHARLOCK
I've seen this crossword clue in The Guardian
C
H
A
R
L
O
C
K
More clues leading to the result CHARLOCK
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 = charlock.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 CHARLOCK mean?
CHARLOCK noun- Any of several yellow-flowered cruciferous weeds of grain fields, especially wild mustard (Brassica kaber).