The most common and most likely answer to this crossword clue is the 7 letter word NIGELLA
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More clues leading to the result NIGELLA
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 = nigella.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 NIGELLA mean?
NIGELLA noun- Any plant of the genus Nigella of about twelve species of annual flowering plants, the blooms of which are generally blue in colour but also found in shades of pink, white and pale purple.
- The seeds of the plant Nigella sativa, used as a culinary spice.