a private eye

private eye

slang A private investigator or detective. "Eye" is a pun on the letter "i" in "investigator." The disgraced police detective moved to another state and started a business as a private eye. It turned out they'd hired a private eye to keep tabs on her after she left the law firm.
See also: eye, private
Farlex Dictionary of Idioms.

a ˌprivate ˈeye

(informal) a detective who is not in the police, but who can be employed to find out information, find a missing person, follow somebody, etc: They hired a private eye to look for more evidence.
See also: eye, private
Farlex Partner Idioms Dictionary
See also:
  • private eye
  • gumshoe
  • hawkshaw
  • shammus
  • shamus
  • catch at
  • catch at (something)
  • a mile off
  • a mile away