The term CELLIST'S NEED is composed of two crossword solutions, ROSIN (5 letters) and BOW (3 letters). ROSIN is a sticky substance made from tree sap that is used to coat the BOW of a stringed instrument, such as a cello. The BOW is a long stick with horsehair stretched across it that is used to draw sound from the strings of a stringed instrument. Both ROSIN and the BOW are essential for a cellist to play their instrument.