Down with faulty acting (5)
I believe the answer is:
fluff
'down' is the definition.
(down can mean bird feathers)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for fluff that I've seen before include "blooper" , "Mistake; soft fibres" , "Botch" , "Trifling" , "picked up by vacuum cleaner?" .)