Duck that is going back to the German (5)
I believe the answer is:
eider
'duck' is the definition.
(eider is a type of duck)
'that is going back to the german' is the wordplay.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'going back' says the letters should be written backwards.
'to the german' becomes 'der' ('the' in German).
'ie' written backwards gives 'ei'.
'ei'+'der'='EIDER'
(Other definitions for eider that I've seen before include "ie Red (anag.)" , "Duck known for down" , "Sea duck with soft down" , "Down's source" , "Variety of duck" .)