Pull some up and see if it is a Radish, leaves look like Radish? If not Radish, best guess is something from the Brassica family. About a hundred different varieties of Brassica, many are hard to tell apart until they grow some and/or flower. Young Rap looks like that also. Crush some leaves and smell it, if is young Basil the smell will tip you off. Mustard, Basil and Rap are all related and have varieties of their own, sometimes hard to tell apart in the early growth stages. Basil has some varieties you'd never now where Basil by looking at them, except they all smell the same.