Labour policy is something that won't be compromised (3,4)
I believe the answer is:
red line
'something that won't be compromised' is the definition.
(a red line is a non-negotiable demand)
'labour policy' is the wordplay.
'labour' becomes 'red' (red is the colour of the Labour Party).
'policy' becomes 'line' (I've seen this before).
'red'+'line'='RED LINE'
'is' is the link.
(Other definitions for red line that I've seen before include "Limit beyond which a party will not go in negotiations" , "Cancel" .)