Colour that's visible on this duck's back (5)
I believe the answer is:
eider
'colour that's visible on this duck's back' is the definition.
'eider' can be an answer for 'duck's' (eider is a type of duck). I am not sure about the rest of the definition.
'on this duck's back' is the wordplay.
'on this' becomes 're' (regarding).
'duck' becomes 'die' (I can't justify this - if you can you should believe this answer much more).
'back' says the letters should be written backwards.
're'+'die'='redie'
'redie' written backwards gives 'EIDER'.
Can you help me to learn more?
(Other definitions for eider that I've seen before include "Down's source" , "Duck with soft feathers" , "I've noted down" , "Source of fine soft down" , "Sea duck with soft down" .)