Eavesdrop on an 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)
'an insect' is the second definition.
(bug is a kind of insect)
'on' acts as a link.
(Other definitions for bug that I've seen before include "Harmful microorganism" , "stripped down motor" , "Computer program error" , "Hidden microphone" , "Computer glitch" .)