Listening device (7)
'listening device' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for bug that I've seen before include "Hidden microphone" , "Secret listening device" , "Software fault" , "Annoy; insect" , "Listening device, insect or electronic fault" .)
(Other definitions for wiretap that I've seen before include "eavesdropping" , "bug" , "Eavesdrop" , "cloak-and-dagger arrangement?" , "sneaky listen" .)