Eavesdropping device (7)
I believe the answer is:
wiretap
'eavesdropping device' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for wiretap that I've seen before include "cloak-and-dagger arrangement?" , "sneaky listen" , "eavesdropping" , "bug" , "Eavesdrop" .)