The most common and most likely answer to this crossword clue is the 4 letter word SIDA
S
I
D
A
Buddy explains!
The straight part
We know that "hempen plant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case hempen plant = sida.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 SIDA mean?
SIDA noun- Any of the flowering plants of the genus Sida in the mallow family