The most common and most likely answer to this crossword clue is the 8 letter word FLEAWORT
I've seen this crossword clue in The Times
F
L
E
A
W
O
R
T
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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 = fleawort.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 FLEAWORT mean?
FLEAWORT noun- Any of various unrelated plants that are supposed to kill or ward off fleas.
- A herb, Plantago psyllium, whose seeds are supposed to resemble fleas