Celestial sightings (4)
I believe the answer is:
'celestial sightings' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ufos that I've seen before include "Initially and formally, flying saucers" , "Supposed alien craft" , "Unauthorised traffic from outer space?" , "Initially, official flying saucers" , "Potential alien craft" .)