Divine musician appearing in a West End theatre (6)
I believe the answer is:
apollo
I believe this is a double definition.
'divine musician' is the first definition.
(god of music)
'a west end theatre' is the second definition.
(apollo theatre)
'appearing in' is the link.
(Other definitions for apollo that I've seen before include "Greek god (twin brother of Artemis)" , "Greek sungod, son of Zeus" , "Greek sun god and leader of the Muses" , "Handsome Greek god" , "Fair youth" .)