The term 'WORN-OUT HORSE' can be broken down into three crossword solutions: NAG (3 letters), PLUG (4 letters), and JADE (4 letters). A NAG is an old or worn-out horse, usually used for riding or pulling a carriage. A PLUG is an old or worn-out horse, typically used for plowing or hauling heavy loads. Finally, JADE is an old or worn-out horse, usually used for racing or show jumping. All three terms refer to an animal that has been overworked and is no longer able to perform its duties.