Notice stain (4)
I believe the answer is:
spot
This clue is a double definition.
'notice' is the first definition.
(spot can mean to notice or observe)
'stain' is the second definition.
(I know that stain can be written as spot)
(Other definitions for spot that I've seen before include "small amount" , "See - speckle" , "Notice; sign of acne" , "See; round mark" , "Fix" .)