Infection forces error (5)
I believe the answer is:
'error' is the definition.
(fluff is a kind of error)
'infection forces' is the wordplay.
'infection' becomes 'flu' (I've seen this before).
'forces' becomes 'ff' (I've seen this in other clues).
(Other definitions for fluff that I've seen before include "Mistake; soft fibres", "blooper", "Deliver badly", "Mess up", "mistake made by some player in action?".)