River duck with a red back (4)
I believe the answer is:
oder
'river' is the definition.
(River Oder in central Europe)
'duck with a red back' is the wordplay.
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'with' says to put letters next to each other.
'back' is a reversal indicator.
'red' back-to-front is 'der'.
'o'+'der'='ODER'
(Other definitions for oder that I've seen before include "School blazer insignia" , "European flower" , "River forming northern part of the German-Polish border" , "River flowing into the Baltic" , "Rode around the European river" .)