Plant or mole (3)
I believe the answer is:
spy
I believe this is a double definition.
'plant' is the first definition.
(a plant can be a person positioned to act as a spy)
'mole' is the second definition.
(I've seen this in another clue)
(Other definitions for spy that I've seen before include "Watch covertly like secret agent" , "Discover" , "Person who secretly collects information as for government" , "Notice" , "One gathers information on enemies" .)