Final answer:
The answer to the riddle is a lawyer, based on the clues provided such as 'cunning linguist,' 'plead and plead,' and reference to briefs. Pleading is a formal legal statement, and lawyers often create and argue legal briefs.
Step-by-step explanation:
The answer to the question, "I am a cunning linguist. I plead and plead for it. What am I?" is a lawyer. This profession involves a person who practices or studies law; an attorney or a counselor. The term 'cunning linguist' suggests someone who is skilled with words, which is a common trait of lawyers. 'Pleading' is a term often used in legal practice, as it refers to the formal statement of the causes of an action or defense. Lawyers are known to craft briefs, which are formal documents presented to a court arguing why one party to a particular case should prevail.