Assuming it is level ground: the best solution is always to build up the level and depth of the topsoil by adding tons of organic matter, compost, manure, whatever. As the level increases the water is more likely to run away somewhere else other than where you are growing things. You need aim for a top-soil depth of at least 50cm on clay.

Horticultural grit in planting holes helps locally only.

If you are on a slope you can try building a soak away at the lowest point - basically a metre deep hole full of rubble.

I am interested to learn that grit only helps locally planted stuff. I've tried to get it in all over, aswell as organic matter.

I've found that grit becomes coated in clay and just becomes part of the clay layer without the organic matter. Sharp sand and fine grit washes down to the clay becoming part of it. It works in planting holes allowing the roots to get deeper and to establish.

In the old days gardens were prepared with a layer of stones below the top soil. Fifty years down the line they form an almost impenetrable barrier in the top of the clay and provide no drainage advantage whatsoever.

If you are just growing vegetables then build deep beds OVER the clay which will gradually enable humus to be taken down by worms and other creatures into the clay below and improve drainage.
With a flower garden or lawn it is of course much more difficult and then I would look at putting in some proper drainage. Mole ploughs which create a horizontal 'drain' by boring a hole through the soil is very effective but only possible if you have access to heavy machinery to use this. Otherwise dig a ditch across the lowest point and install proper clay drainage pipes.