What is Articled Clerk?

Legal Definition
An articled clerk is someone who is studying to either be an accountant or lawyer. In doing so they are put under the supervision of someone already in the profession, usually for two years. This can be compared as being an intern for a company, the only difference is once the two years is over the trainee becomes the trainer. Trainees are obligated to sign a contract agree to the term of being an articled clerk. The articled clerk signs a contract, known as "articles of clerkship", committing to a fixed period of employment. Wharton's Law Lexicon defines an articled clerk as "a pupil of a solicitor, who undertakes, by articles of clerkship, continuing covenants, mutually binding, to instruct him in the principles and practice of the profession". The contract is with a specific partner in the firm and not with the firm as a whole.

Apprentice architects can also be articled. Henry Percy Adams articled to Britwen Binyon (1846-1909) architect.
-- Wikipedia
Legal Definition
In English law. A clerk bound to serve in the office of a solicitor in consideration of being instructed in the profession. This is the general acceptation of the term; but it is said to be equally applicable to other trades and professions. Reg. v. Reeve, 4 Q. B. 212.
-- Black's Law Dictionary
Legal Definition
A lawyer's clerk bound to him by contract providing for his service and for his instruction in law.
-- Ballentine's Law Dictionary