supergrass

noun

an informer whose information implicates a large number of people in terrorist activities or other major crimes

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Examples from the Web for supergrass

Contemporary Examples of supergrass

The new names were handed over by a suspect turned "supergrass".
Diana's Boyfriend Fears Phone Was Hacked
Tom Sykes
March 16, 2013
All this is small beer compared with the supergrass that BCL Burton Copeland may turn out to be.
Murdoch’s Lawyers Turn on Him
Sam Bungey
August 18, 2011