Reset switch for old music system (6)
I believe the answer is:
stereo
'music system' is the definition.
(I've seen this before)
'reset switch for old' is the wordplay.
'switch' is an anagram indicator.
'for' says to put letters next to each other.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'reset' is an anagram of 'stere'.
'stere'+'o'='STEREO'
(Other definitions for stereo that I've seen before include "Twin-channel sound" , "Sound coming from different directions" , "In-depth" , "Record player" , "Tape recorder" .)