British liner rebuilt in once-walled city (6)
I believe the answer is:
berlin
'walled city' is the definition.
(I've seen this in another clue)
'british liner rebuilt in' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'rebuilt' indicates an anagram.
'in' indicates putting letters inside.
'liner' with letters rearranged gives 'erlin'.
'b' inserted into 'erlin' is 'BERLIN'.
'once' acts as a link.
(Other definitions for berlin that I've seen before include "Irving . . . . . . , great American song-writer" , "German capital" , "Isaiah, say?" , "Composer < in carriage < in European capital" , "Possibly Isaiah" .)