Stereo alternative (4)
I believe the answer is:
'stereo alternative' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for quad that I've seen before include "Rectangular area with buildings on all sides" , "One of four children born at one birth" , "Rectangular area surrounded by buildings" , "Sort of bike" , "Square (abbr)" .)