/ (ˈsuːpəˌɡrɑːs) /
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
The new names were handed over by a suspect turned "supergrass".
All this is small beer compared with the supergrass that BCL Burton Copeland may turn out to be.