nark, copper's nark -- (an informer or spy working for the police)
narc, nark, narcotics agent -- (a lawman concerned with narcotics violations)
Overview of verb nark
The verb nark has 2 senses
annoy, rag, get to, bother, get at, irritate, rile, nark, nettle, gravel, vex, chafe, devil -- (cause annoyance in; disturb, especially by minor irritations; "Mosquitoes buzzing in my ear really bothers me"; "It irritates me that she never closes the door after she leaves")