Alternatively, a threesome taking round choral piece (8)
I believe the answer is:
oratorio
'choral piece' is the definition.
(I've seen this in another clue)
'alternatively a threesome taking round' is the wordplay.
'alternatively' becomes 'or'.
'threesome' becomes 'trio' (both can mean a group of three).
'taking' means one lot of letters goes inside another.
'round' becomes 'o' (o is a circular letter).
'trio' going around 'o' is 'torio'.
'or'+'a'+'torio'='ORATORIO'
(Other definitions for oratorio that I've seen before include "Music for voices and orchestra with biblical theme" , "I, too, roar at sacred music" , "Eg, Walton's Belshazzar's Feast" , "Work such as Handel's Messiah" , "Music for many performers" .)