The most common and most likely answer to this crossword clue is the 6 letter word SPURGE
I've seen this crossword clue in The Daily Telegraph
S
P
U
R
G
E
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 = spurge.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 SPURGE mean?
SPURGE noun- Any plant of the genus Euphorbia, a diverse genus of over 2,000 species.
- To emit foam; to froth; said of the emission of yeast from beer during fermentation.