Mortise filler is a type of woodworking tool used to fill the gap between two pieces of wood that have been joined together with a mortise and TENON joint. The term 'TENON' is the solution to the crossword, which is a type of joint where one piece of wood has a protruding end that fits into a notch in the other piece of wood. Mortise filler is used to fill the gap between the two pieces of wood, creating a strong and secure joint.