Old boy damaged violin, showing forgetfulness (8)
I believe the answer is:
oblivion
'forgetfulness' is the definition.
(oblivion is a kind of forgetfulness)
'old boy damaged violin' is the wordplay.
'old boy' becomes 'ob'.
'damaged' indicates anagramming the letters.
'violin' with letters rearranged gives 'livion'.
'ob'+'livion'='OBLIVION'
'showing' acts as a link.
(Other definitions for oblivion that I've seen before include "Total forgetfulness -- limbo" , "State of unawareness" , "State of being totally unconscious" , "Insensibility" , "Unconsciousness - nothingness" .)