The term 'FASTEN, AT SEA' can be solved by two crossword solutions: BELAY (5 letters) and LASH (4 letters). BELAY is a nautical term that refers to the process of securing a rope or cable to a cleat or belaying pin. This is done to prevent the rope from slipping or becoming loose. LASH is another nautical term that refers to the process of tying two objects together with rope or cord. This is often used to secure items on a boat or ship while at sea.