Alien craft
'alien 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 "Initially, official flying saucers" , "Initially and formally, flying saucers" , "Unauthorised traffic from outer space?" , "Supposed alien craft" , "Initially, formal flying saucers" .)
(Other definitions for ufo that I've seen before include "One unexplained overhead" , "Potential alien ship" , "(Possibly) alien craft" , "initially and formally,a flying saucer" , "Initially, a formal flying saucer" .)
(Other definitions for flying saucers that I've seen before include "UFOs" , "Extraterrestrial objects, apparently observed by some people" .)