The term MATTRESS FILLING can be summarized by the crossword solution KAPOK, which is a five-letter word. KAPOK is a type of fiber that is derived from the seed pods of the KAPOK tree, which is native to tropical regions of the world. It is often used as a filling for mattresses, pillows, and other items due to its lightweight and buoyant properties. KAPOK is also naturally resistant to dust mites and mold, making it a popular choice for those with allergies.