Lock-up party on old musical (9)
I believe the answer is:
brigadoon
'musical' is the definition.
'lock-up party on old' is the wordplay.
I cannot really see how this works, but
'party' could be 'do' (do is a kind of party) and 'do' is located in the answer.
'on' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for brigadoon that I've seen before include "Lerner and Loewe musical" , "Musical entertainment" , "Musical set in Scotland" , "Musical set in Ireland" .)