Two-for-one is a term used to describe a special offer or discount in which two items are sold for the price of one. In this particular crossword, the solutions SPARE, INTEGER, PRIME, and DIGIT all relate to the concept of two-for-one. SPARE is an adjective meaning extra or surplus, INTEGER is a mathematical term for a whole number, PRIME is another mathematical term for a number that can only be divided by itself and one, and DIGIT is a single symbol used to make up a number. All of these terms can be related to the concept of two-for-one in some way.