The term FASTENS (DOWN) can be solved by two crossword solutions: BATTENS (7 letters) and SECURES (7 letters). BATTENS is a noun that refers to thin strips of wood or metal used to hold down the edges of a roof, door, or window. SECURES is a verb that means to make something safe or secure. Both words are related to the idea of fastening something in place, which is the definition of the term FASTENS (DOWN).