The term 'Back, to a Shellback' is a reference to a sailor who has crossed the equator and is thus considered an experienced sailor. The crossword solution for this term is AFT, which stands for 'AFTER'. AFT is used to describe the position of something that is located behind or toward the stern of a ship. Therefore, the term 'Back, to a Shellback' can be interpreted as a sailor returning to their experienced status AFTER crossing the equator.