In Cologne, a restaurant that's most convenient (7)
I believe the answer is:
nearest
'most convenient' is the definition.
(I've seen this in another clue)
'in cologne a restaurant' is the wordplay.
'in' indicates a hidden word.
'NEAREST' can be found hidden inside 'cologne a restaurant'.
'that's' acts as a link.
(Other definitions for nearest that I've seen before include "Relations" , "Stingiest" , "Most local" , "Most closely related" , "Closest to hand" .)