Socialist takes nothing back to river (4)
I believe the answer is:
oder
'to river' is the definition.
(River Oder in central Europe)
'socialist takes nothing back' is the wordplay.
'socialist' becomes 'red' (socialism is often associated with the colour red).
'takes' means one lot of letters go next to another.
'nothing' becomes 'O' (O resembles 0).
'back' says the letters should be written in reverse.
'red'+'o'='redo'
'redo' reversed gives 'ODER'.
(Other definitions for oder that I've seen before include "waterway" , "flowing water" , "European banker" , "Polish/German river" , "European flower" .)