chain letter vs letter

chain letter and letter both are nouns.

Word	Noun	Adjective	Verb	Adverb
chain letter	Yes	No	No	No
letter	Yes	No	Yes	No

As nouns, letter is a hypernym of chain letter; that is, letter is a word with a broader meaning than chain letter:

Other hypernyms of chain letter include missive.

chain letter (noun)	letter (noun)
a letter that is sent successively to several people	a written message addressed to a person or organization
	the conventional characters of the alphabet used to represent speech
	an award earned by participation in a school sport
	a strictly literal interpretation (as distinct from the intention)
	owner who lets another person use something (housing usually) for hire

chain letter (verb)	letter (verb)
	mark letters on or mark with letters
	set down or print with letters
	win an athletic letter