The most common and most likely answer to this crossword clue is the 9 letter word INFORMANT
I've seen this crossword clue in The Sunday Telegraph
I
N
F
O
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A
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T
More clues leading to the result INFORMANT
- Source social worker doing well at work?
- One tells where a schoolkid may be, with a set of books
- Singer doing well, joining Queen perhaps
- Grass worker who's playing well?
- Books on playing well on the earliest of artificial grass
- Hip name in design for person shopping
- He tips off six-footer currently playing well, perhaps
- Source of popular fashion followed by worker?
- Currently doing well, insect in grass
- Grass due to face time trapping bloke
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 = informant.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 INFORMANT mean?
INFORMANT noun- One who relays confidential information to someone, especially to the police; an informer.
- A native speaker who acts as a linguistic reference for a language being studied. The informant demonstrates native pronunciation, provides grammaticality judgments regarding linguistic well-formedness, and may also explain cultural references and other important contextual information.