Complex sentence: Nick Fury told Maria Hill that there was a Skrull impersonating a S.H.I.E.L.D. agent, and in the episode "Prisoners of War", Clay is one of the prisoners on the Skrull ship, proving that a Skrull is posing as him on Earth.
Split into simpler sentences: Nick Fury told Maria Hill that there was a Skrull impersonating a S.H.I.E.L.D. agent. In the episode "Prisoners of War", Clay is one of the prisoners on the Skrull ship proving that a Skrull is posing as him on Earth.