Coating of a bird? (5)
I believe the answer is:
layer
'coating' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for layer that I've seen before include "plate?" , "Thickness of material or earth - relay it" , "Lamina" , "One of several thicknesses" , "Source of eggs" .)