Prayer day that is coming round, at first chanted with a backing (5,3)
I believe the answer is:
agnus dei
'prayer day that is coming round at first chanted with a backing' is the definition.
'Agnus Dei' can be an answer for 'prayer' (Agnus Dei is an example). I'm not certain of the remainder of the definition.
'prayer day that is coming round' is the wordplay.
'prayer' becomes 'agnus' (I can't justify this - if you can you should believe this answer much more).
'day' becomes 'd' (abbreviation for day).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'coming round' says the letters should be written in reverse.
'ie' back-to-front is 'ei'.
'agnus'+'d'+'ei'='AGNUS DEI'
Can you help me to learn more?
(Other definitions for agnus dei that I've seen before include "Part of the ordinary" , "religious piece" , "sacrificial offering" , "Section of mass" , "Part of Latin Mass" .)