The term 'SECRET STUFF' can be broken down into two crossword solutions: ARCANA (6 letters) and CODES (5 letters). ARCANA is a noun that refers to mysterious or esoteric knowledge, while CODES are sets of rules or regulations used to keep information confidential. Both terms are related to the concept of secrecy, and are often used in contexts such as cryptography, espionage, and other forms of covert operations.