Catch sight of blemish (4)
I believe the answer is:
spot
I believe this clue is a double definition.
'catch sight of' is the first definition.
(to spot is to notice or catch sight of)
'blemish' is the second definition.
(I know that blemish can be written as spot)
(Other definitions for spot that I've seen before include "point on snooker table" , "Difficult situation" , "Notice; mark" , "Dirty blemish" , "Espy; mark" .)