Soak the clams in water for about six hours, changing the water every two hours or so, to expel the sand inside the shells. Drain and rinse several times until the liquid is clear and free from traces of sand.
Heat the dashi in a pot.
Ladle half a cup of hot dashi into a bowl and stir in the miso paste until free from lumps.
When the dashi is simmering, add the cleaned clams and cook just until the shells open.