That’s what the police earn after 11 pm.
Explanation: This joke is a pun that plays on the term “copper,” which is slang for police, and the phrase “nitrate,” which sounds like “night rate.” The humor comes from the idea that police might earn a different rate of pay (“night rate”) for their work during nighttime hours, particularly after 11 PM. By combining “copper” and “nitrate,” the punchline cleverly links the police to their nighttime duties.