underwear vs underclothes

underwear and underclothes both are nouns.

Word	Noun	Adjective	Verb	Adverb
underwear	Yes	No	No	No
underclothes	Yes	No	No	No

As nouns, underwear and underclothes are synonyms defined as:

underwear and underclothes: undergarment worn next to the skin and under the outer garments

Other synonyms of underwear include underclothing.

underwear (noun)	underclothes (noun)
undergarment worn next to the skin and under the outer garments	undergarment worn next to the skin and under the outer garments