Sound system (2-2)
'sound system' is the definition.
(hi-fi is a kind of sound system)
This is all the clue.
(Other definitions for hi-fi that I've seen before include "High fidelity" , "Good quality sound system, in short" , "High-quality sound equipment" , "Stereo" , "Good sound reproduction, in short" .)
'sound system' is the definition.
(stereo is a kind of sound system)
This is the entire clue.
(Other definitions for stereo that I've seen before include "Twin-channel sound" , "Tape recorder" , "Audio system, hi-fi" , "Record-player" , "System that gives a three-dimensional sound" .)