I had heavy clay soil in a garden and agree with Happymarion, all manure, mushroom compost bulky matter you can get and dig in will help breal it down.I found runner beans loved it as it retains moisture and has good nutrients.
As for shade, foxgloves,hardy geraniums(not peragoniums), pulmonaria, green hellebores, wild violets, forget me nots, brunneria, lily of the valley. lamium, lysimachia(creeping jenny, but get the yellow one not the green as it is invasive), periwinkle, aquilegia, asters, euphorbia, violas, skimmia.........and many more like or tolerate shade. All these I have grown and once watered in and established are as tough as old boots!!