Finish in German city that's more fashionable (8)
I believe the answer is:
trendier
'more fashionable' is the definition.
('trendy' can be similar in meaning to 'fashionable')
'finish in german city' is the wordplay.
'finish' becomes 'end' ('end' can be a synonym of 'finish').
'in' is an insertion indicator.
'german city' becomes 'trier' (I've seen this before).
'end' going into 'trier' is 'TRENDIER'.
'that's' acts as a link.
(Other definitions for trendier that I've seen before include "More stylish, up-to-the-minute" , "More up with the latest fashions" , "More up-to-date and with it" , "more fashion-conscious?" , "cooler" .)