The term SIGN OF A SELLOUT is composed of two crossword solutions, CROWD and SRO. CROWD is a 5-letter word meaning a large number of people gathered together in one place. SRO is a 3-letter acronym for Standing Room Only, which is a phrase used to describe a situation where all the seats in a venue are taken and there is no more room to sit. Together, these two terms can be used to describe a situation where a large number of people have gathered to attend an event, indicating that it has been a great success.