A Grecian throng reworked religious song (9,5)
I believe the answer is:
gregorian chant
'religious song' is the definition.
(Gregorian chant is an example)
'a grecian throng reworked' is the wordplay.
'reworked' indicates an anagram.
'a'+'grecian'+'throng'='agrecianthrong'
'agrecianthrong' with letters rearranged gives 'GREGORIAN CHANT'.
(Other definitions for gregorian chant that I've seen before include "variety of plainsong" , "Medieval plainsong" .)