Border broken up in united Germany (4)
I believe the answer is:
edge
'border' is the definition.
(synonyms)
'broken up in united germany' is the wordplay.
'broken up in' indicates a hidden word (the answer is split across words in the clue).
'EDGE' is hidden in the letters of 'united germany'.
(Other definitions for edge that I've seen before include "Lip; advantage" , "Rim or sharp side of knife" , "Some bordering" , "'Margin, rim (4)'" , "Sharpness of mind" .)