canned chopped clams
Cooked, finely chopped clams packed in their own juice, in a can.
Look for them in the canned fish aisle of larger supermarkets, often as whole baby clams, at Italian delis as canned vongole, or in Asian grocers; the briny juice carries much of the flavour and is used along with the meat.
Substitute:
Canned whole clams or frozen clam meat, roughly chopped, work just as well; with fresh clams, steam them open briefly and use the cooking liquid in place of the canned juice, while mussels taste stronger and somewhat different.