A flat fee is a fixed amount of money that is charged for a service or product. In the context of this crossword, the term 'flat fee' could refer to RENT and security deposit. RENT is a fixed amount of money that is paid to a landlord in exchange for the use of a property. A security deposit is an amount of money that is paid by a tenant to a landlord as a form of insurance against any damage to the property during the tenancy.