Flying saucer flier (5)
I believe the answer is:
'flying saucer flier' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for alien that I've seen before include "Being from another world" , "Belonging to a foreign country" , "24a [ESTRANGE]" , "repugnant" , "extraneous" .)