Part a: what is the meaning of the phrase “unfair which fairly doth excel” as it is used in shakespeare’s “sonnet 5”?

This poem discusses the passage of time and time's effect on beauty.

When Shakespeare writes "unfair which fairly doth excel," he says that time (which once helped a child age into beauty) will then take that beautiful adult and make him or her old. That which now "excels" will eventually be made unfair.

Basically, time was good to the child, by making the child a beautiful adult, but soon time will be "unfair" and "play the tyrant" to the beautiful adult, who currently "excels," by making him or her old.