Long piece of writing on the origin of some belief system (6)
I believe the answer is:
screed
'long piece of writing' is the definition.
(I've seen this before)
'on the origin of some belief system' is the wordplay.
'on' means one lot of letters go next to another.
'the origin of' indicates taking the first letters.
'belief system' becomes 'creed' (I've seen this in another clue).
The first letter of 'some' is 's'.
's' next to 'creed' is 'SCREED'.
(Other definitions for screed that I've seen before include "Layer applied to floor" , "lengthy document" , "Mortar layer on concrete floor for smooth finish" , "extensive text" , "Mortar used for levelling" .)