The term 'Cheer Opener' is a job title for someone who starts a cheerleading routine. The crossword solutions for this term are SIS (3 letters) and ONETWOTHREEFOUR (15 letters). SIS is an abbreviation for Sister, which is often used as a term of endearment among cheerleaders. ONETWOTHREEFOUR is a reference to the common cheerleading chant 'One, two, three, four, let's hear it for the team!' which is often used to start a cheer.