Something blown up in stereo box (4)
I believe the answer is:
oboe
'something blown' is the definition.
(oboe is a kind of wind instrument)
'up in stereo box' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'in' indicates a hidden word.
'eobo' is hidden in the letters of 'stereo box'.
'eobo' in reverse letter order is 'OBOE'.
(Other definitions for oboe that I've seen before include "Instrument used in 1d" , "Orchestral instrument" , "issuer of notes" , "Walk with exaggerated movements" , "source of notes" .)