Groups of performers overlooking core musical (4)
I believe the answer is:
cats
'musical' is the definition.
(I've seen this before)
'groups of performers overlooking core' is the wordplay.
'groups of performers' becomes 'casts'.
'overlooking core' means to remove the middle letters.
'casts' with its middle taken out is 'CATS'.
(Other definitions for cats that I've seen before include "1981 Lloyd Webber musical" , "Pets - musical" , "Musical from TS Eliot" , "Feline musical by Andrew Lloyd Webber" , "Feline mammals" .)