Sci-fi craft (4)
I believe the answer is:
ufos
'sci-fi craft' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ufos that I've seen before include "Mysterious phenomena" , "Initially, official flying saucers" , "Potential alien craft" , "Initially and formally, flying saucers" , "Initially, formal flying saucers" .)