Organ accompanying old musical (6)
I believe the answer is:
oliver
'musical' is the definition.
(musical based on Oliver Twist)
'organ accompanying old' is the wordplay.
'organ' becomes 'liver' (liver is a kind of organ).
'accompanying' is a charade indicator (letters next to each other).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'liver' put after 'o' is 'OLIVER'.
(Other definitions for oliver that I've seen before include "Dickensian role?" , "Musical (with a Twist!)" , "Paladin (with Roland)" , "A Cromwell, a Twist" , "Roland companion; Bath biscuit" .)