Quiet voice - security feature of Windows? (7)
I believe the answer is:
shutter
'security feature of windows?' is the definition.
'quiet voice' is the wordplay.
'quiet' becomes 'sh'.
'voice' becomes 'utter' (I've seen this before).
'sh'+'utter'='SHUTTER'
(Other definitions for shutter that I've seen before include "Hinged window blind" , "Camera mechanism" , "Moveable window screen" , "camera component" , "Hinged window cover" .)