Cleanskin

Law enforcement uses the word cleanskin to refer to an undercover operative whose identity is unknown to the police forces he or she is tasked to find information about. This is usually because such an agent has not conducted any prior undercover activity.
The phrase got more use with a slightly different meaning in the United Kingdom following the London bombings of 7 July 2005. The four bombers involved in those bombings were reported in the press to be "cleanskins", according to police sources, meaning that their profiles did not fit what bombers are usually like.