Old boy smashed violin in stupor (8)
I believe the answer is:
oblivion
'stupor' is the definition.
(thesaurus)
'old boy smashed violin in' is the wordplay.
'old boy' becomes 'ob' (abbreviation for a private school 'old boy').
'smashed' is an anagram indicator.
'in' means one lot of letters goes inside another.
'violin' is an anagram of 'livion'.
'ob' going inside 'livion' is 'OBLIVION'.
(Other definitions for oblivion that I've seen before include "State of total unconciousness" , "State of being forgotten and disregarded" , "State of being mentally withdrawn or blank" , "Last of man's 22 20 [SEVEN AGES]" , "State of being altogether forgotten" .)