clerk vs mail clerk

clerk and mail clerk both are nouns.

Word	Noun	Adjective	Verb	Adverb
clerk	Yes	No	Yes	No
mail clerk	Yes	No	No	No

As nouns, mail clerk is a hyponym of clerk; that is, mail clerk is a word with a more specific, narrower meaning than clerk:

clerk: an employee who performs clerical work (e.g., keeps records or accounts)
mail clerk: a clerk in a post office

clerk (noun)	mail clerk (noun)
an employee who performs clerical work (e.g., keeps records or accounts)	a clerk in a post office
a salesperson in a store

clerk (verb)	mail clerk (verb)
work as a clerk, as in the legal business