Worthless audio device has vital second application (6-5)
I believe the answer is:
mickey-mouse
'worthless' is the definition.
(mickey-mouse can mean of poor quality)
'audio device has vital second application' is the wordplay.
'audio device' becomes 'mic' (microphone).
'has' means one lot of letters go next to another.
'vital' becomes 'key' (I've seen this before).
'second' becomes 'mo' (both are short periods of time).
'application' becomes 'use' (I've seen this in another clue).
'mic'+'key'+'mo'+'use'='MICKEY-MOUSE'
(Another definition for mickey-mouse that I've seen is " Ineffective".)