The term 'USE A FOIL' can be broken down into two crossword solutions: FENCE (5 letters) and DUEL (4 letters). FENCE is a structure made of posts and rails, usually made of wood or metal, used to enclose an area. A DUEL is a formal fight between two people, usually with weapons, in order to settle a dispute. In this context, the term 'USE A FOIL' could refer to the practice of fencing, which involves two opponents using foils to DUEL each other.