plainclothesman vs investigator

plainclothesman and investigator both are nouns.

Word	Noun	Adjective	Verb	Adverb
plainclothesman	Yes	No	No	No
investigator	Yes	No	No	No

As nouns, investigator is a hypernym of plainclothesman; that is, investigator is a word with a broader meaning than plainclothesman:

Other hypernyms of plainclothesman include detective, police detective, tec.

plainclothesman (noun)	investigator (noun)
a detective who wears civilian clothes on duty	someone who investigates
	a scientist who devotes himself to doing research
	a police officer who investigates crimes