Secret listening devices (4)
I believe the answer is:
bugs
'secret listening devices' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for bugs that I've seen before include "Insects or defects in computer programs" , "Small crawling creatures or disease-germs" , "taps" , "Secret listening devices" , "Creepy-crawlies in computers" .)