Undercover means working in secret under a false name and a false story. An officer who spends a year as a delivery driver to get inside a gang; a reporter who takes a job in a warehouse to see what happens on the floor.
It sits in front of the noun almost always — an undercover officer, an undercover operation, an undercover reporter — and it works as an adverb too: she went undercover for eight months. It is about identity, not simply secrecy; a secret meeting is not an undercover meeting, because nobody there is pretending to be someone else. The false identity itself is the cover, which is why an agent's cover can be blown. Keep it apart from its neighbours: underground is for hidden movements and resistance, covert is for secret operations in general. And there is no hyphen — undercover is one word.
1adjectiveundercoverDone or working in secret, using a false identity.
“An undercover officer had been in the warehouse for months.”
2adverbsecretIn secret and under a false identity.
“Two detectives went undercover as buyers.”
Too few real examples of this word to estimate which sense is commonest.
Undercover is exactly what it looks like: under plus cover. The cover is the story an agent hides behind — a job, a name, a reason for being there — and to work undercover is to work beneath one.
From under + cover.
Etymology from Wiktionary, CC BY-SA