Pretty German city, that is (6)
I believe the answer is:
bonnie
'pretty' is the definition.
(I've seen this before)
'german city that is' is the wordplay.
'german city' becomes 'bonn' (city in Germany).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'bonn'+'ie'='BONNIE'
(Other definitions for bonnie that I've seen before include "One of two gangsters" , "The Young Pretender" , "Leader of the 1745 Jacobite uprising" , "- - Prince Charlie" , "Pretty" .)