Green’s Dictionary of Slang

a lump of phlegm.

1955		E. Hoagland Cat Man 5: [T]he old men kept the clams in their mouths; they didn’t spit them on the floor.
1970		in DARE.
1981		H. Gould Fort Apache, The Bronx 90: ‘That was some clam you laid on him.’ [...] Murphy wiped his chin.
2002		R. St Amand ‘Sweat Flavoured Gummi’ Journal at Blogspot.com 🌐 You know what I do like? The term ‘clam.’ It’s a very Massachusetts colloquial term for hawking up some glob from within.