plain clothes vs civilian clothing

plain clothes and civilian clothing both are nouns.

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

As nouns, plain clothes and civilian clothing are synonyms defined as:

plain clothes and civilian clothing: ordinary clothing as distinguished from uniforms, work clothes, clerical garb, etc.

Other synonyms of plain clothes include civilian dress, civilian garb.

plain clothes (noun)	civilian clothing (noun)
ordinary clothing as distinguished from uniforms, work clothes, clerical garb, etc.	ordinary clothing as distinguished from uniforms, work clothes, clerical garb, etc.