Smear or smudge (4)
I believe the answer is:
blur
This clue is a double definition.
'smear' is the first definition.
(I know that smear can be written as blur)
'smudge' is the second definition.
(I know that smudge can be written as blur)
(Other definitions for blur that I've seen before include "fade" , "Hazy or indistinct representation" , "Ill-defined smear" , "Make hazy" , "Make or become indistinct" .)