Notice a mark (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)
'a mark' is the second definition.
(I know that spot is a more specific form of the action mark)
(Other definitions for spot that I've seen before include "Discern - mark" , "Notice; pimple" , "Informally, catch sight of" , "Detect" , "Pimple; place" .)