Colonel's gone wrong in German city (7)
I believe the answer is:
cologne
'german city' is the definition.
(I've seen this before)
'colonel's gone wrong in' is the wordplay.
'colonel' becomes 'col' ().
'wrong' indicates anagramming the letters.
'in' means one lot of letters goes inside another.
'gone' anagrammed gives 'ogne'.
'col' placed into 'ogne' is 'COLOGNE'.
(Other definitions for cologne that I've seen before include "Scent - German city" , "smells nice" , "German river port" , "Refreshing scent" , "Town" .)