Depressed so let fly the feathers (4,4)
I believe the answer is:
cast down
'depressed' is the definition.
(cast down can mean depressed or downbeat)
'let fly the feathers' is the wordplay.
'let fly' becomes 'cast' (I've seen this in another clue).
'the feathers' becomes 'down' (down can mean bird feathers).
'cast'+'down'='CAST DOWN'
'so' acts as a link.
(Other definitions for cast down that I've seen before include "result of fewer actors getting parts?" , "Gloomy" , "Sadden" , "in need of encouragement" , "Dishearten" .)