The most common and most likely answer to this crossword clue is the 5 plus 8 letter word LADYS BEDSTRAW
I've seen this crossword clue in The Daily Telegraph
L
A
D
Y
S
B
E
D
S
T
R
A
W
More clues leading to the result LADYS BEDSTRAW
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 = ladys bedstraw.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.