A blemish you'll notice (4)
I believe the answer is:
spot
This clue could be a double definition.
'a blemish' is the first definition.
(I know that blemish can be written as spot)
'you'll notice' is the second definition.
'spot' can be an answer for 'notice' (spot can mean to notice or observe). I'm not sure about the 'you'll' bit.
Can you help me to learn more?
(Other definitions for spot that I've seen before include "Difficult situation" , "Small mark; observe" , "Pickle" , "Fix" , "See 5" .)