The crossword solution for the term 'Theatre Backup' is 'QUEUE'. QUEUE is a term used to describe a line of people waiting for something, such as a performance or service. It is derived from the French word 'cuer', which means 'TAIL'. In the theatre, a QUEUE is often used to organize the audience before a show. The backup in this case refers to the extra people who are waiting in the QUEUE in case there are any cancellations or no-shows.