An informant, especially one who informs against others, often for compensation.(noun)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "informer" in a sentence
"To turn these facts into "Socialist icon who became an informer" is itself (sorry, but it's irresistible)"
"He, however, declined the risk, declaring it was impossible to capture the schooner with boats, and as she was a remarkably fast sailer, she was sure to escape; should the enterprise not succeed, he would become known as the informer, and be no longer able to act as pilot in the Bahama Channel."
"Maj Klopper was shown a document in which one of the informers accidentally signed under his real name, which Col de Kock had deleted and put in a note saying that the informer was a covert agent."