Socialist policy which may be a deal breaker (3,4)
I believe the answer is:
red line
'a deal breaker' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'socialist policy' is the wordplay.
'socialist' becomes 'red' (red can informally mean a socialist).
'policy' becomes 'line' (the party line is a party's policy on something).
'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" .)