Lots of actors originally starred in a musical (5)
I believe the answer is:
casts
'lots of actors' is the definition.
(more than one cast of actors)
'originally starred in a musical' is the wordplay.
'originally' suggests taking the first letters.
'in' indicates putting letters inside.
'a musical' becomes 'cats' (Andrew Lloyd Webber musical).
The first letter of 'starred' is 's'.
's' put inside 'cats' is 'CASTS'.
(Other definitions for casts that I've seen before include "Allocates parts" , "Throws" , "Assigns an actor to a role" , "Eg puts" , "groups of actors" .)