The crossword solution for the term 'close up tight' is 'SEAL'. A SEAL is an object or device that is used to close off an opening, often to prevent air, water, dust, or other contaminants from entering. Seals are commonly used in a variety of industries, including food and beverage, automotive, and pharmaceuticals. Seals can also be used to secure documents, such as contracts, and to authenticate the identity of individuals.