Crossword Genius

Socialist policy which may be a deal breaker (3,4)

Ross

I believe the answer is:

red line

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

'socialist policy' is the wordplay.
'socialist' becomes 'red' (red can informally mean a socialist).
'policy' becomes 'line' (as in the party line).
'red'+'line'='RED-LINE'

'which may be' acts as a link.

Can you help me to learn more?

(Other definitions for red line that I've seen before include "Limit beyond which a party will not go in negotiations" , "Cancel" .)

I've seen this clue in The Independent.
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