Violently liberates northern walled city once (4,6)
I believe the answer is:
east berlin
'walled city once' is the definition.
I can't judge whether this definition defines the answer.
'violently liberates northern' is the wordplay.
'violently' indicates anagramming the letters (letters are violently disrupted).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'liberates' is an anagram of 'eastberli'.
'eastberli'+'n'='EAST BERLIN'
Can you help me to learn more?
(Other definitions for east berlin that I've seen before include "Former Communist zone" , "area once walled off" , "Soviet sector once" , "old part of capital" .)