The term 'HORSE OPERA SOUND' is a crossword solution that can be broken down into two words: 'CLOP' (4 letters) and 'SOUND'. 'CLOP' is the SOUND of a horse's hooves hitting the ground, and 'SOUND' refers to the SOUND of a horse opera, which is a type of musical drama featuring horses and cowboys. Horse operas were popular in the early 20th century, and the SOUND of a horse's hooves was often used as a soundtrack for these films.