German doctor returns in a cab (6)
I believe the answer is:
hansom
'a cab' is the definition.
(I've seen this in another clue)
'german doctor returns' is the wordplay.
'german' becomes 'hans' (I've seen this in other clues).
'doctor' becomes 'mo' (medical officer).
'returns' says the letters should be written in reverse.
'mo' in reverse letter order is 'om'.
'hans'+'om'='HANSOM'
'in' acts as a link.
(Other definitions for hansom that I've seen before include "Old horse cab sounds attractive" , "One-horse cab" , "transport from the old days" , "Two-wheeled one-horse carriage" , "Two-wheeled horse-drawn covered carriage" .)