Notice small stain (4)
I believe the answer is:
spot
I believe this is a double definition.
'notice' is the first definition.
(spot can mean to notice or observe)
'small stain' is the second definition.
(I've seen this in another clue)
(Other definitions for spot that I've seen before include "Notice - place" , "See; location" , "make out" , "See - blemish" , "Identify" .)