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