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
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Examples from the Web for supergrass
Contemporary Examples of 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.