Crossword Genius

Bowie number succeeded in the future, 'Tom' having faded out? (6)

Ross

I believe the answer is:

sorrow

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'out?' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

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

Can you help me to learn more?

(Other definitions for sorrow that I've seen before include "Lamenting" , "Deep distress" , "'Distress, sadness (6)'" , "Deep sadness" , "Expression of grief" .)

I've seen this clue in The Guardian.
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