Creatures from another planet (6)
I believe the answer is:
aliens
'creatures from another planet' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for aliens that I've seen before include "Creatures from another planet" , "Strange forms of life" , "Outsiders" , "Life-forms assumed to exist beyond Earth" , "Foreigners or creatures from outer space" .)