Not mono (6)
I believe the answer is:
stereo
'not mono' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for stereo that I've seen before include "Music-playing equipment giving depth of sound" , "A 19 player" , "Method of sound reproduction" , "Three-dimensional sound effect, in short" , "Sound coming from different directions" .)