Performing groups succeeded in musical (5)
I believe the answer is:
casts
'performing groups' is the definition.
(I've seen this in another clue)
'succeeded in musical' is the wordplay.
'succeeded' becomes 's' (genealogical abbreviation).
'in' means one lot of letters goes inside another.
'musical' becomes 'cats' (Andrew Lloyd Webber musical).
's' placed inside 'cats' is 'CASTS'.
(Other definitions for casts that I've seen before include "Throws" , "Assigns an actor to a role" , "Allocates parts" , "Eg puts" , "groups of actors" .)