After battle, Napoleon evacuated a part of Austria (6)
I believe the answer is:
vienna
'part of austria' is the definition.
('Vienna' is part of 'austria')
'after battle napoleon evacuated a' is the wordplay.
'after' says to put letters next to each other.
'battle' becomes 'vie'.
'evacuated' means to remove the middle letters.
'napoleon' with its centre removed is 'nn'.
'vie' next to 'nn' is 'vienn'.
'vienn'+'a'='VIENNA'
(Other definitions for vienna that I've seen before include "European city for New Year's Day concert" , "Austria's capital city" , "Capital city of Austria" , "Austrian capital" , "On the Danube, it was home to many great composers" .)