Initially supposed to come from outer space (3)

I believe the answer is:

ufo

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'space' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for ufo that I've seen before include "Something mysterious" , "Mysterious sighting" , "Space enigma" , "Mysterious object in sky" , "Potential spaceship" .)

I've seen this clue in The Globe and Mail.

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