The phrase "undercover policeman" refers to a police officer who pretends to be someone else in order to secretly gather information and catch criminals without them knowing. Full definition
Gerard Johnson's gritty British thriller follows a corrupt, coke - snorting undercover policeman (the excellent Peter Ferdinando, who also starred in Johnson's previous film, 2009's Tony) as he becomes embroiled in a... (nme.com)
And what about undercover policemen? (planet3.org)
An undercover policeman has revealed to Premier he left his job in California to rescue children from... More (premierchristianity.com)