One of those in a costly pile-up? (4)
I believe the answer is:
debt
'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.
Can you help me to learn more?
(Other definitions for debt that I've seen before include "Something owed" , "Amount owing" , "Amount owed" , "state of obligation" , "sense of gratitude" .)