The most common and most likely answer to this crossword clue is the 8 letter word BEDSTRAW
I've seen this crossword clue in The Guardian
B
E
D
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More clues leading to the result BEDSTRAW
Buddy explains!

The straight part
We know that "herbaceous plant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case herbaceous plant = 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.
What does BEDSTRAW mean?
BEDSTRAW noun- Any plant of herb genus Galium of the madder family with small pointed leaves and hairy stems with small, white or yellow flowers
- Any plant of the genus Cruciata.
- Straw put into a bed