Hi-fi system (6)
I believe the answer is:
stereo
'hi-fi system' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for stereo that I've seen before include "Musical sound with two speakers" , "With twin audio channels" , "Of sound equipment, having two speakers" , "A 19 player" , "Sound - not mono" .)