Infection forces error (5)
I believe the answer is:
fluff
'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).
'flu'+'ff'='FLUFF'
(Other definitions for fluff that I've seen before include "Error" , "Make a mistake" , "Mess up" , "picked up by vacuum cleaner?" , "blooper" .)