Green’s Dictionary of Slang

1. (US black) any police officer [an agent properly works only for the FBI and is not, as such, a police officer].

1965		B. Jackson Get Your Ass in the Water (1974) 206: The room was filled with reefer smoke, / one agent got high and soon told a joke.
1968		‘Sl. of Watts’ in Current Sl. III:2.

2. (N.Z. prison) a new gang recruit, still in training.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 5/2: agent n. 1 a gang recruit, a young gangmember training for his patch.

3. (N.Z. gang/prison) an individual, possibly not a member, who oversees the operation of a gang while its senior members are in prison.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 5/2: agent n. 2 a person, not necessarily a gang-member, employed to oversee the general management of a gang while its key members are in prison.

4. (N.Z. gang/prison) an informer.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 5/2: agent n. 3 an informer.