Celestial sightings (4)
I believe the answer is:
ufos
'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" .)