Extraterrestrials (6)
I believe the answer is:
aliens
'extraterrestrials' is the definition.
('alien' can be a synonym of 'extraterrestrial')
This is all the clue.
(Other definitions for aliens that I've seen before include "Non-nationals - perhaps from Mars?" , "Beings from another planet" , "Creatures from another planet" , "Immigrants" , "Extra-terrestrial beings" .)