Song sung initially in tree (5)
I believe the answer is:
psalm
'song' is the definition.
(I've seen this before)
'sung initially in tree' is the wordplay.
'initially' 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 initial letter of 'sung' is 's'.
's' placed into 'palm' is 'PSALM'.
(Other definitions for psalm that I've seen before include "Church hymn" , "Devotional hymn or song" , "A song which praises" , "Lamps (anag) -- song" , "Jewish hymn" .)