Extraterrestrial (5)
'extraterrestrial' is the definition.
(synonyms)
This is all the clue.
(Other definitions for alien that I've seen before include "Not native" , "Offensive" , "Foreign (from outer space?)" , "Little green man, perhaps" , "Creature from another planet" .)
'extraterrestrial' is the definition.
(a lifeform from Mars)
This is the entire clue.
(Other definitions for martian that I've seen before include "Invader in The War of the Worlds (H. G. Wells)" , "seeing little green man?" , "Relating to one planet" , "Sci-fi character" , "Sci-fi alien" .)