Rub together is a phrase that is used to describe the action of two objects being moved against each other in a back-and-forth motion. The term is often used to describe the sound of two objects being rubbed together, such as when two hands are clapped together. In this context, the crossword solution GNASH is an appropriate answer, as it is a verb meaning to grind or grate one's teeth together, which is a type of rubbing.