Crossword Genius

What was foreshadowed by Percival Lowell initially before bowing out? (5)

Ross

I believe the answer is:

pluto

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'out?' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for pluto that I've seen before include "Disney character - Underworld god - planet" , "Planet beyond Neptune" , "Planet discovered in 1930" , "wartime acronym" , "Solar system body" .)

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