Xenomorph (5)
I believe the answer is:
alien
'xenomorph' is the definition.
(I know that xenomorph can be written as Alien)
This is all the clue.
(Other definitions for alien that I've seen before include "Creature from another planet" , "extraneous" , "Foreign; extraterrestrial" , "Little green man, perhaps" , "coming from overseas" .)