Set pulsating disc on first audio device (6)
I believe the answer is:
stereo
'audio device' is the definition.
'set pulsating disc on first' is the wordplay.
'set pulsating disc' becomes 'stere' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'on first' becomes 'o' (sst letter of 'on').
'stere'+'o'='STEREO'
Can you help me to learn more?
(Other definitions for stereo that I've seen before include "Sound - not mono" , "Means of amplification of sound through two or more channels" , "A 19 player" , "Three-dimensional sound effect, in short" , "Description of record" .)