Sacred performance changing rate, or otherwise (9,5)
I believe the answer is:
gregorian chant
'sacred performance' is the definition.
'changing rate or otherwise' is the wordplay.
'otherwise' indicates an anagram.
'changing'+'rate'+'or'='changingrateor'
'changingrateor' is an anagram of 'GREGORIAN CHANT'.
(Other definitions for gregorian chant that I've seen before include "Medieval plainsong" , "variety of plainsong" .)