Song starts off copying hymn, adapting normal tune (5)
I believe the answer is:
chant
'song' is the definition.
(I've seen this before)
'starts off copying hymn adapting normal tune' is the wordplay.
'starts off' suggests taking the first letters.
The first letters of 'copying hymn adapting normal tune' is 'CHANT'.
(Other definitions for chant that I've seen before include "Repeated rhythmic song" , "Intone - recite" , "Type of group singing" , "Song (on the terraces?)" , "Repetitive song" .)