Song sung originally in tree! (5)
I believe the answer is:
psalm
'song' is the definition.
(I've seen this before)
'sung originally in tree' is the wordplay.
'originally' says to take the initial letters.
'in' means one lot of letters goes inside another.
'tree' becomes 'palm' (palm tree is a kind of tree**).
The first letter of 'sung' is 's'.
's' put within 'palm' is 'PSALM'.
(Other definitions for psalm that I've seen before include "Old Testament hymn" , "Church hymn" , "Song - hymn - prayer" , "Sacred poem attributed to David" , "Devotional hymn or song" .)