Notice, espy (4)
I believe the answer is:
spot
I believe this clue is a double definition.
'notice' is the first definition.
(spot can mean to notice or observe)
'espy' is the second definition.
(synonyms)
(Other definitions for spot that I've seen before include "Catch sight of mark in pots" , "Detect" , "point on snooker table" , "Notice - place" , "See; pimple" .)