The term 'BILL TACK-ON' is a crossword solution that consists of two words. The first word, 'BILL', is a 5 letter word meaning 'RIDER'. A RIDER is someone who rides ON the back of an animal, such as a horse or a camel, or in a vehicle, such as a car or a boat. The second word, 'TACK-ON', is a 7 letter word meaning 'to add ON'. To add ON means to attach something extra to something else, usually for a specific purpose. Therefore, the term 'BILL TACK-ON' can be interpreted as 'adding a RIDER to something'.