clam vs greenback

clam and greenback both are nouns.

Word	Noun	Adjective	Verb	Adverb
clam	Yes	No	Yes	No
greenback	Yes	No	No	No

As nouns, greenback is a hypernym of clam; that is, greenback is a word with a broader meaning than clam:

clam (noun)	greenback (noun)
burrowing marine mollusk living on sand or mud; the shell closes with viselike firmness	a piece of paper money (especially one issued by a central bank)
flesh of either hard-shell or soft-shell clams
a piece of paper money worth one dollar

clam (verb)	greenback (verb)
gather clams, by digging in the sand by the ocean