You are most likely looking for Misoshiru, Japanese Miso Soup to which various ingredients can be added, much like various ingredients may be added to a basic chicken soup.
You can strain the scallions, wakame, and tofu out if you want it to resemble the dish your husband had. I'd still cook them in to impart flavor though.
Simply add the freshest clams you can find. Cooking time for the clams will depend on their size and how many you use...
Miso Soup "Misoshiru"
1/4 - 1/2packagethinly sliced Tofu (silken or soft) - one centimeter dices
2-4cupsdashi soup (basic soup stock) [recipe below]
2tablespoons Miso (red miso)
1tablespoonthinly sliced scallions
1. Bring soup stock to a boil. Add Aka Miso and stir until dissolved and to taste.
2. Add Tofu, Wakame flakes and scallion to soup. Heat soup again over low heat for about 1/2 minutes. Stop heating before boiling.
3. Remove from heat and pour into soup bowls.
1 T Bonito flakes
2Kombu (kelp) 5 inch in length
1. Soak Kombu in water for one hour in large sauce pan.
2.Bring kombu-water to simmer over medium heat about 5 minutes. Remove kombu from water and discard just before full boil or Kombu will impart undesirable flavor.
3.Boil the Kombu-water, then add Bonito flakes. Boil 1 minute.
4.Turn off heat and let the kombu-water sit undisturbed for about 2 minutes or until the bonito flakes come down and stay at bottom.
5.Using a fine mesh strainer, strain kombu-water into a medium pot. Discard Bonito.
6.Serve immediately or store in fridge up to 3~4 days. Can freeze in ice trays for future use.
Enjoy! I hope it's a good starting point for you to try your hand...