Notice blemish (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)
'blemish' is the second definition.
(spot is a kind of blemish)
(Other definitions for spot that I've seen before include "See; pimple" , "jam > stain" , "Small round mark" , "clock" , "Mark - position - perceive" .)