It would be right out of order to call this a flower (4)
I believe the answer is:
oder
This could be a double definition.
'it would be right out of order to call this a' is the first definition.
The definition suggests a singular noun which matches the answer.
'flower' is the second definition.
(River Oder in central Europe)
Can you help me to learn more?
(Other definitions for oder that I've seen before include "River flowing into the Baltic" , "German banker" , "River that, with the Neisse, forms the German-Polish border" , "Rode around the European river" , "flowing water" .)