You could mistake it for a rather small and weedy cow parsley flower, but the Pignut is a separate species of umbellifer.  It forms well-spaced groups in the long grass.
Each plant has a small tuber on the roots, rather like the kernal of a hazel nut.  Pigs like them - hence the name - but they are also edible by humans and quite tasty. 
Recognition clue: the leaves of the pignut plant are very finely divided.