The cheek of some musicians! (5)
I believe the answer is:
brass
I believe this clue is a double definition.
'the cheek' is the first definition.
(brass neck can mean cheek or insolence)
'some musicians' is the second definition.
(brass section of an orchestra)
'of' is the link.
(Other definitions for brass that I've seen before include "Alloy of copper and zinc" , "Alloy; money" , "An alloy; part of orchestra" , "Zinc-copper alloy - cheek" , "Baggage" .)