Sentence:  Bobby Kennedy believed the Mafia had lots of interests in and people in Cuba; he was absolutely convinced that they had left <WSD>stay-behinds</WSD> in Cuba and all they had to do was tap into those <WSD>stay-behinds</WSD>. 
Meaning of the word marked with <WSD> tags:  An agent who lives in a foreign country, and can be activated in certain circumstances.