walk-in vs secret agent

walk-in and secret agent both are nouns.

Word	Noun	Adjective	Verb	Adverb
walk-in	Yes	Yes	No	No
secret agent	Yes	No	No	No

As nouns, secret agent is a hypernym of walk-in; that is, secret agent is a word with a broader meaning than walk-in:

walk-in: an operative who initiates his own defection (usually to a hostile country) for political asylum
secret agent: a person secretly employed in espionage for a government

Other hypernyms of walk-in include defector, deserter, intelligence agent, intelligence officer, operative.

walk-in (noun)	secret agent (noun)
a small room large enough to admit entrance	a person secretly employed in espionage for a government
an assured victory (especially in an election)
an operative who initiates his own defection (usually to a hostile country) for political asylum
person who walks in without having an appointment

walk-in (adjective)	secret agent (adjective)
(of e.g. closets or refrigerators) extending very far enough back to allow a person to enter