A tireless worker is someone who works without rest or pause, often with great dedication and effort. In this crossword, the term 'tireless worker' is represented by the word 'ROBOT', which is a five-letter word. Robots are machines that are programmed to perform certain tasks, and they are often used in place of humans to do repetitive and laborious work. They are known for their tireless work ethic, as they can work for long periods of time without needing rest or breaks.