Weary socialist starts warning (3,4)
I believe the answer is:
red flag
'warning' is the definition.
(I've seen this before)
'weary socialist starts' is the wordplay.
'weary' becomes 'flag' (both can mean to tire).
'socialist' becomes 'red' (socialism is often associated with the colour red).
'starts' means one lot of letters go next to another.
'flag' put after 'red' is 'RED FLAG'.
(Other definitions for red flag that I've seen before include "Warning signal" , "left-wing message?" , "Left-wing symbol" , "The ... ..., anthem of the British Labour Party" , "Sign of danger - sign of socialism" .)