Blue feature of a feather-eater (4,2,3,5)
I believe the answer is:
down in the mouth
'blue' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for down in the mouth that I've seen before include "consequence of eating unprepared duck?" , "then spitting feathers?" , "evidence of poor pheasant plucking?" , "Hangdog" , "characteristic of a retriever on a duck hunt?" .)