Long document beginning to support belief system (6)
I believe the answer is:
screed
'long document' is the definition.
(I've seen this before)
'beginning to support belief system' is the wordplay.
'beginning to' indicates taking the first letters.
'belief system' becomes 'creed' (I've seen this in another clue).
The first letter of 'support' is 's'.
's'+'creed'='SCREED'
(Other definitions for screed that I've seen before include "Tiresomely long letter about creeds" , "Long speech or text" , "Lengthy statement" , "A long written effusion" , "Layer of mortar to level a floor" .)