A more thorough explanation:

A Clerk of the Privy Seal is a historical officer who was responsible for preparing documents for the Lord Privy Seal. The Privy Seal was used to seal royal letters and other documents that did not require the Great Seal of the Realm. This office was abolished in 1884.

A Clerk of the Signet was an officer who kept the privy signet and attended the sovereign's principal secretary. The signet was used to seal royal letters and other documents not requiring the Great Seal of the Realm. The office was abolished in England in 1851.

A clerkship is a type of internship in which a law student or recent law-school graduate assists a lawyer or judge with legal writing, research, and other tasks. In many common-law jurisdictions, recent law-school graduates are required to complete clerkships as a condition of admission to the bar. Historically, a law student's employment as an attorney's apprentice before gaining admission to the bar was also called a clerkship. Until shortly before WWII, in many states a person could be admitted to the bar without attending law school merely by passing the bar exam.

For example, a recent law school graduate may work as a clerk for a judge, helping with legal research and writing opinions. This experience can be valuable in preparing the graduate for a career as a lawyer. In the past, a law student may have worked as an apprentice for a practicing attorney, learning the skills necessary to become a lawyer.