Poles expanded into German city (5)
I believe the answer is:
essen
This clue could be a double definition.
'poles expanded into german' is the first definition.
I know that 'german' relates to this answer.
'city' is the second definition.
(Essen is an example)
(Other definitions for essen that I've seen before include "city in the Ruhr valley" , "Krupp HQ (Ger.)" , "Industrial city of northwest Germany" , "Main industrial city of the Ruhr" , "German industrial centre" .)