We have really heavy clay here, yet we're surrounded by some really lovely meadows. People are surprised, but they shouldn't be. Althought some clay soils can have fertility higher than ideal, our soil is distinctly mediocre from an agricultural point of view - and pretty good for meadow making. These soils are typically pretty neutral in terms of pH, and do sometimes get waterlogged, but that doesn't matter. All I would say is don't seed them too late in the autumn, else the seed will just rot if left in standing water.