plain clothes vs clothing

plain clothes and clothing both are nouns.

Word	Noun	Adjective	Verb	Adverb
plain clothes	Yes	No	No	No
clothing	Yes	No	No	No

As nouns, clothing is a hypernym of plain clothes; that is, clothing is a word with a broader meaning than plain clothes:

plain clothes: ordinary clothing as distinguished from uniforms, work clothes, clerical garb, etc.
clothing: a covering designed to be worn on a person's body

Other hypernyms of plain clothes include article of clothing, habiliment, vesture, wear, wearable.

plain clothes (noun)	clothing (noun)
ordinary clothing as distinguished from uniforms, work clothes, clerical garb, etc.	a covering designed to be worn on a person's body