Notice agent (3)
I believe the answer is:
spy
I believe this clue is a double definition.
'notice' is the first definition.
(thesaurus)
'agent' is the second definition.
(spy is a kind of agent)
(Other definitions for spy that I've seen before include "Mole, agent" , "Watch covertly like secret agent" , "One gathers information on enemies" , "Catch sight of" , "Notice" .)