Eavesdrop on insect (3)
I believe the answer is:
bug
I believe this is a double definition.
'eavesdrop' is the first definition.
(bugging is a kind of eavesdroping)
'insect' is the second definition.
(I know that insect can be written as bug)
'on' acts as a link.
(Other definitions for bug that I've seen before include "Software fault" , "An enthusiast" , "Harmful microorganism" , "Irritate" , "Computer glitch" .)