When depressed, one stops listening (4,6)
I believe the answer is:
mute button
'depressed one stops listening' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.
'when' is the wordplay.
I cannot quite see how this works, but
'when' could be 'on' (I've seen this in other clues) and 'on' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for mute button that I've seen is " microphone switch?".)