Clay soil is simply soil with too much clay in it, relative to the sand and humus content.
You can restore the balance by adding sand, and/or humus in the form of peat or farmyard manure.
You will first need to break up the clay to enable proper mixing to take place.
Clay soil is heavy and lumpy because the particles stick together.Clay soil is heavy and lumpy because the particles stick together.
Clay soil drains badly because of the particles sticking together and preventing the passage of water. Clay soil is often waterlogged.
Plants find it difficult to grow through such a heavy and hard medium.

Plants suitable for Clay Soil:

This is not an exhaustive list by any means.

I must stress that clay soil is not an ideal growing medium.