Fluff or muff (5)
I believe the answer is:
error
I believe this is a double definition.
'fluff' is the first definition.
(I've seen this in another clue)
'muff' is the second definition.
(I've seen this in another clue)
(Other definitions for error that I've seen before include "A mistake or inaccuracy" , "Trial and ___" , "Mistake or fault" , "heresy" , "Blunder, slip" .)