Audio system (6)
I believe the answer is:
stereo
'audio system' is the definition.
(stereo is a kind of audio system)
This is the entire clue.
(Other definitions for stereo that I've seen before include "Sound transmitted through two speakers" , "Audio system, hi-fi" , "Of sound equipment, having two speakers" , "Sound transmission from two sources through two channels" , "When sound comes through two channels" .)