supergrass

No, it's not vegetation or any form of cannabis. A "supergrass" in British slang is an informer whose testimony points to many suspects; to "grass on" someone is to tattle on him.

"An Al Qaeda supergrass held in the U.S. is to be shown pictures of the terror suspects arrested in Britain in the hope that he can identify them."

Link: 'Terror ringleader' is brilliant NHS doctor | News | This is London