They're out of this world (6)
I believe the answer is:
'they're out of this world' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for aliens that I've seen before include "Outsiders" , "Creatures from another planet" , "Non-nationals - perhaps from Mars?" , "Foreigners or extraterrestrials" , "Extra-terrestrial beings" .)