Slip down (5)
I believe the answer is:
fluff
I believe this clue is a double definition.
'slip' is the first definition.
(thesaurus)
'down' is the second definition.
(thesaurus)
(Other definitions for fluff that I've seen before include "Light downy substance - mistake?" , "blooper" , "Mistake; soft fibres" , "Deliver badly" , "Trifling" .)