The most common and most likely answer to this crossword clue is the 6 letter word DARNEL
I've seen this crossword clue in The Daily Telegraph
D
A
R
N
E
L
More clues leading to the result DARNEL
Buddy explains!

The straight part
We know that "grass" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case grass = darnel.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 DARNEL mean?
DARNEL noun- A species of ryegrass, Lolium temulentum, often found in wheat fields and often host to a fungus intoxicating to humans and animals.
- Various species of Lolium, especially as a weed in wheat fields.