The term 'relative of a glockenspiel' can be answered with the word 'celesta', which is a seven-letter word. The celesta is a musical instrument that was invented in 1886 by Auguste Mustel. It is a type of keyboard instrument that produces a sound similar to a glockenspiel, but with a softer and more delicate tone. The celesta is often used in classical music, film scores, and popular music.