The term 'OX-CESSORY' is a combination of two words, 'OX' and 'ACCESSORY'. An OX is a bovine animal, usually a bull or cow, that is used for plowing and other agricultural tasks. An ACCESSORY is an item that is added to something else to make it more attractive or functional. Therefore, the crossword solution 'YOKE' (4 letters) is an OX-cessory, as it is an item that is used to attach two oxen together in order to pull a plow or other heavy object.