The term 'FEATURE OF A MODERN ZOO' can be summarized by the crossword solution 'MOAT', which is a four-letter word. A MOAT is a deep, wide ditch that is filled with water and is typically found surrounding a castle or other fortification. In modern zoos, moats are used to separate animals from visitors, providing a safe barrier between them. Moats also provide a naturalistic environment for the animals, allowing them to feel more at home in their enclosure.