Freshwater duck with a red back (4)
I believe the answer is:
oder
'freshwater duck with a red back' is the definition.
I can't judge whether this definition defines the answer.
'duck with a red back' is the wordplay.
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'with' means one lot of letters go next to another.
'back' is a reversal indicator.
'red' backwards is 'der'.
'o'+'der'='ODER'
Can you help me to learn more?
(Other definitions for oder that I've seen before include "waterway" , "Polish/German river" , "European flower" , "German banker" , "German river that empties into the Baltic Sea" .)