Wine by sink left mark (3,4)
I believe the answer is:
'left mark' is the definition.
(symbol associated with the left)
'wine by sink' is the wordplay.
'wine' becomes 'red' (red is a type of wine).
'by' means one lot of letters go next to another.
'sink' becomes 'flag' (flagging is a kind of sinking).
(Other definitions for red flag that I've seen before include "Emblem of socialism" , "Instruction to halt" , "A flyer" , "Warning signal" , "Sign of danger - sign of socialism" .)