Green’s Dictionary of Slang

(US Und.) an informer.

1938		D. Runyon ‘Cemetery Bait’ in Runyon on Broadway (1954) 524: He is a singer, at heart.
1946		N.Y. Times 15 Dec. SM16: Singer: an informer or stool pigeon.
1956		‘Toney Betts’ Across the Board 104: Gross [...] turned singer against policemen. He told the Grand Jury, the cops were his partners in crime.