'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" .)