The term ONE UNDERCOVER is a combination of two words, the first being 'ONE' and the second being 'UNDERCOVER'. The first word, 'ONE', is used to describe a single person or thing. The second word, 'UNDERCOVER', is used to describe someone who is working in secret or UNDERCOVER for an organization or government. In this case, the term ONE UNDERCOVER is referring to a SPY, which is a person who works secretly for a government or organization to gather information.