Spooner's escaped torment? That signals danger! (3,4)

I believe the answer is:

red flag

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'danger' is the definition.
Both the answer and definition are singular nouns.
I also know that 'danger' relates to this answer.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for red flag that I've seen before include "Warning sign" , "Socialist banner?" , "Socialist symbol" , "A flyer" , "The ... ..., anthem of the British Labour Party" .)

I've seen this clue in The Telegraph.

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