underclass vs social class

underclass and social class both are nouns.

Word	Noun	Adjective	Verb	Adverb
underclass	Yes	Yes	No	No
social class	Yes	No	No	No

As nouns, social class is a hypernym of underclass; that is, social class is a word with a broader meaning than underclass:

underclass: the social class lowest in the social hierarchy
social class: people having the same social, economic, or educational status

Other hypernyms of underclass include class, socio-economic class, stratum.

underclass (noun)	social class (noun)
the social class lowest in the social hierarchy	people having the same social, economic, or educational status

underclass (adjective)	social class (adjective)
belonging to the lowest and least privileged social stratum