The term 'INGREDIENT IN A DARK 'N' STORMY' is referring to the alcoholic beverage known as a Dark 'n' Stormy, which is made with dark RUM and ginger beer. The crossword solution for this term is RUM, which is a type of distilled alcoholic beverage made from sugarcane byproducts, such as molasses, or directly from sugarcane juice. It is an essential ingredient in a Dark 'n' Stormy, and is also used in many other cocktails.