The crossword solution for the term 'What Mares Eat, In Song' is OATS. This is a reference to the popular children's song 'Old MacDonald Had a Farm', which includes the lyric 'And on his farm he had some OATS, ee-i-ee-i-o'. The song is about a farmer and his animals, and the mare is one of the animals mentioned in the song. Therefore, the answer to the crossword clue is OATS, as this is what mares eat in the song.