Sacred hymn initially sung in tree (5)
I believe the answer is:
psalm
'sacred hymn' is the definition.
(I've seen this in another clue)
'initially sung in tree' is the wordplay.
'initially' suggests taking the first letters.
'in' indicates putting letters inside.
'tree' becomes 'palm' (palm is a kind of tree).
The first letter of 'sung' is 's'.
's' placed into 'palm' is 'PSALM'.
(Other definitions for psalm that I've seen before include "Old Testament hymn" , "Devotional song/hymn" , "Religious song" , "Church hymn" , "Sacred song from Old Testament" .)