Sci-fi alien (7)
I believe the answer is:
martian
'sci-fi alien' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for martian that I've seen before include "Extraterrestrial" , "Being from another planet" , "Invader in The War of the Worlds (H. G. Wells)" , "Sci-fi character" , "Relating to one planet" .)