secret police vs police

secret police and police both are nouns.

Word	Noun	Adjective	Verb	Adverb
secret police	Yes	No	No	No
police	Yes	No	Yes	No

As nouns, police is a hypernym of secret police; that is, police is a word with a broader meaning than secret police:

secret police: a police force that operates in secrecy (usually against persons suspected of treason or sedition)
police: the force of policemen and officers

Other hypernyms of secret police include constabulary, law, police force.

secret police (noun)	police (noun)
a police force that operates in secrecy (usually against persons suspected of treason or sedition)	the force of policemen and officers

secret police (verb)	police (verb)
	maintain the security of by carrying out a patrol