Alien spaceship (6,6)
I believe the answer is:
flying saucer
'alien spaceship' is the definition.
(I know that flying saucer is a type of unidentified flying object)
This is all the clue.
(Other definitions for flying saucer that I've seen before include "UFO?" , "Extraterrestrial visitor" , "Phenomenon" , "observed when bull enters china shop?" .)