Son in tree produces sacred poem (5)
I believe the answer is:
psalm
'sacred poem' is the definition.
(I've seen this in another clue)
'son in tree' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'in' means one lot of letters goes inside another.
'tree' becomes 'palm' (palm is a kind of tree).
's' going within 'palm' is 'PSALM'.
'produces' acts as a link.
(Other definitions for psalm that I've seen before include "Sacred Biblical song" , "Lamps for a sacred song" , "Lamps (anag) -- song" , "Religious song" , "Sacred poem (written by King David?)" .)