Zoom lens that's employed covertly, by the way (5,6)
I believe the answer is:
speed camera
'that's employed covertly by the way' is the definition.
(something hidden by a road or way)
'zoom lens' is the wordplay.
'zoom' becomes 'speed' (I've seen this in another clue).
'lens' becomes 'camera lens' (camera lens is a kind of lens).
'speed'+'camera'='SPEED CAMERA'
(Other definitions for speed camera that I've seen before include "flashy source of government income" , "Road safety device" , "Roadside feature" , "Roadside detector" , "Roadside deterrent" .)