Notice blemish (4)
I believe the answer is:
I believe this clue is a double definition.
'notice' is the first definition.
(spot can mean to notice or observe)
'blemish' is the second definition.
(I know that blemish can be written as spot)
(Other definitions for spot that I've seen before include "See; pimple" , "jam > stain" , "Small round mark" , "clock" , "Mark - position - perceive" .)