By the colour, you'll see how old the feathers are (7)
I believe the answer is:
plumage
'feathers are' is the definition.
'plumage' can be an answer for 'feathers' (I've seen this in another clue). I'm not sure about the 'are' bit.
'by the colour you'll see how old' is the wordplay.
'by' means one lot of letters go next to another.
'the colour you'll see' becomes 'plum' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'how old' becomes 'age' (I've seen this before).
'plum' put next to 'age' is 'PLUMAGE'.
'the' acts as a link.
Can you help me to learn more?
(Other definitions for plumage that I've seen before include "Birds' clothing" , "coat" , "One's feathers" , "A bird's feathers, may be fancy" , "Cover for diver" .)