What startling information does Mr. Cruncer have concerning the death of Roger Cly, and how does this information help Carton?

Carton tries to use what he knows about Barsad (the fact that he is currently employed by the Republican government under a false name but was formerly employed by the English government-which would lead the French government to believe he is a spy) to help free Darnay. He threatens to denounce Barsad, adding that he recognizes the man with whom Barsad was talking as Roger Cly. Barsad tries to claim that Cly is dead and had a funeral back in London, but Jerry Cruncher interjects, saying that he looked in that coffin, and there was no body in it. Despite the fact that he grows defensive when asked why he knows this, Cruncher sticks by his assertion, and Barsad gives up and agrees to help Carton. Carton asks to have a final word alone with Barsad.