Was a medieval clerk a girl?

1st answer:

No.There weren't any "clerks". Things were grown or made by individuals and sold by them at the weekly market or to the clergy/manor.

2nd Answer:

Medieval clerks were not usually women, but some were.

A very interesting article called "Women in Medieval Guilds," by Nicky Saunders, has a list of jobs medieval women are recorded to have had, and clerk was one of them.

There are links below to that article and to a related question on the things medieval women did.