The most common and most likely answer to this crossword clue is the 4 letter word OATS
I've seen this crossword clue in Independent
O
A
T
S
More clues leading to the result OATS
- Promises to put out hard breakfast cereal?
- A time to tuck into extra-large breakfast dish?
- Animals beheaded as food for horse
- German leaving animals out to grass
- In the grass, succeeded getting sexual satisfaction
- River craft have no British cereal
- Seeds explorer’s broadcast
- Little beasts, knocking off street food
- Seeds that could be sown in the wild?
- Vessels unloading the first crop
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 = oats.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 OATS mean?
OATS noun- Widely cultivated cereal grass, typically Avena sativa.
- Any of the numerous species, varieties, or cultivars of any of several similar grain plants in genus Avena.
- (usually as plural) The seeds of the oat, a grain, harvested as a food crop.
- A simple musical pipe made of oat-straw.
- A mass of oat plants (genus Avena, especially Avena sativa).
- Seeds of an oat plant, especially prepared as food.