Musical old organ (6)
I believe the answer is:
oliver
'musical' is the definition.
(I've seen this before)
'old organ' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'organ' becomes 'liver' (liver is a kind of organ).
'o'+'liver'='OLIVER'
(Other definitions for oliver that I've seen before include "-- Reed, actor" , "Dickens-based musical" , "Fictional boy who famously asked for more" , "One taking pride in food" , "Dickensian musical" .)