The most common and most likely answer to this crossword clue is the 6 letter word SNITCH
I've seen this crossword clue in The Daily Telegraph
S
N
I
T
C
H
More clues leading to the result SNITCH
- Steal from school, carrying can back
- Inform of allergic reaction to tin?
- Serbian contracted skin problem from grass
- Headlouse seen in school, briefly? I’m telling!
- Finally plays in long grass
- Partners longing for grass
- Poles must have urge to squeal
- Vacant sign seen on long grass
- Poles must anticipate urge to grass
- Idiot in school who betrays classmate?
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 = snitch.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 SNITCH mean?
SNITCH noun- A thief.
- An informer, usually one who betrays his group.
- A nose.
- A tiny morsel.
- To inform on, especially in betrayal of others.
- To contact or cooperate with the police for any reason.
- To steal, quickly and quietly.