... and in Berlin, nothing is free (4)
I believe the answer is:
undo
'is free' is the definition.
(as in undoing someone's chains)
'and in berlin nothing' is the wordplay.
'and in berlin' becomes 'und' ('and' in German).
'nothing' becomes 'o' (looks like zero - 0).
'und'+'o'='UNDO'
(Other definitions for undo that I've seen before include "Cancel -- annul -- reverse" , "Overturn" , "Open buttons or ruin" , "Loosen or reverse" , "Relax" .)