A small part to play -- 'Tra-la!' -- with this song (5)
I believe the answer is:
psalm
'a small part to play tra-la with this song' is the definition.
'psalm' can be an answer for 'song' (I've seen this before). I am not sure about the rest of the definition.
'small part to play tra-la with this song' is the wordplay.
I cannot quite understand how this works, but
'small' could be 's' (abbreviation - e.g. clothes size) and 's' is found within the answer.
'part' could be 'palm' (palm is a kind of part) and 'palm' is found in the remaining letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for psalm that I've seen before include "Sacred song from Old Testament" , "OT hymn" , "Palms (anag.)" , "Old Testament lyrical poem or prayer" , "Religious song" .)