The crossword solution for the term 'mattress accessory' is 'PAD, ' which is a three-letter word. A mattress PAD is an accessory that is placed on top of a mattress to provide extra cushioning and comfort. It can also help protect the mattress from dirt, dust, and other debris. Mattress pads come in a variety of materials, such as cotton, memory foam, and latex, and can be found in different sizes and thicknesses.