Old organ used for stage musical (6)
I believe the answer is:
oliver
'for stage musical' is the definition.
(musical based on Oliver Twist)
'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'
'used' is the link.
(Other definitions for oliver that I've seen before include "Musical based on the story of Dickens boy" , "One taking pride in food" , "Musical (with a Twist!)" , "Chef" , "Lionel Bart musical" .)