Know that, without his ring, the diamond-seller ran amok and went completely off the rails. (5,5)

I believe the answer is:

train wreck

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'know that without his ring the diamond-seller ran amok and went completely off the rails' is the definition.
The definition suggests a singular noun which matches the answer.

This is the entire clue.

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(Other definitions for train wreck that I've seen before include "disaster" , "likely result of hard Brexit" .)

I've seen this clue in The Irish Times.

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