Potential spaceship (3)
I believe the answer is:
ufo
'potential spaceship' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ufo that I've seen before include "Airborne enigma" , "Initially, a formal flying saucer" , "initially and formally,a flying saucer" , "Potential flying saucer" , "Potential alien craft" .)