A bird's complaint (6)
I believe the answer is:
thrush
I believe this clue is a double definition.
'a bird's' is the first definition.
(thrush is a kind of bird)
'complaint' is the second definition.
(thrush is a kind of health problem)
(Other definitions for thrush that I've seen before include "Mouth fungus or songbird" , "Common British songbird" , "sprue" , "Throstle" , "Songbird with brownish back and speckled breast" .)