The phrase "doeth bit" is an archaic term that is not often used in modern English. It is generally interpreted to mean "does a little," and the closest synonym that comes to mind is "accomplishes something small." However, there are a variety of other phrases or idioms that can be used in its place, depending on the context. Some possible substitutes include "makes progress," "takes a step," "makes a dent," "takes a stab at it," and "puts in effort." These alternative phrases can help to convey a similar idea in a more contemporary and accessible way.