Crossword Genius

One of those in a costly pile-up? (4)

Ross

I believe the answer is:

debt

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'one' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

I can't explain the rest of the clue.

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(Other definitions for debt that I've seen before include "Something owed" , "Amount owing" , "Amount owed" , "state of obligation" , "sense of gratitude" .)

I've seen this clue in the Evening Standard.
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