In a tree start to sing a song (5)
I believe the answer is:
psalm
'a song' is the definition.
(I've seen this before)
'in a tree start to sing' is the wordplay.
'in' means one lot of letters goes inside another.
'a tree' becomes 'palm' (palm is a kind of tree).
'start to' suggests taking the first letters.
The initial letter of 'sing' is 's'.
'palm' going around 's' is 'PSALM'.
(Other definitions for psalm that I've seen before include "Religious song" , "Song - hymn - prayer" , "Old Testament lyrical poem or prayer" , "Lamps for a sacred song" , "Sacred song of praise" .)