Spy's device (3)
I believe the answer is:
tap
'spy's device' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tap that I've seen before include "Gentle stroke" , "Make use of; water from it" , "Knock lightly; a valve" , "One can be hot or cold?" , "Get money from, strike" .)