Famous actor originally involved in musical (7)
I believe the answer is:
olivier
'famous actor' is the definition.
(Sir Laurence Olivier)
'originally involved in musical' is the wordplay.
'originally involved' becomes 'i' (I am not sure about this - if you are sure you should believe this answer much more).
'in' is an insertion indicator.
'musical' becomes 'oliver' (musical based on Oliver Twist).
'i' put within 'oliver' is 'OLIVIER'.
Can you help me to learn more?
(Other definitions for olivier that I've seen before include "First actor to become a life peer" , "Noted actor" , "Laurence --, first actor to be made a life peer" , "N Messiaen, French composer" , "Former acting lord" .)