Some plaster saint and the belief system at its base (6)
I believe the answer is:
screed
'base' is the definition.
(I have seen 'concrete base' mean 'screed' so perhaps 'base' could also mean 'screed')
'some plaster saint and the belief system at its' is the wordplay.
I cannot really see how this works, but
'saint' could be 's' and 's' is located in the answer.
'belief' could be 'creed' (creed is a kind of belief) and 'creed' is present in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for screed that I've seen before include "concrete base" , "Layer applied to floor" , "Layer of mortar to level a floor" , "Mortar layer on concrete floor for smooth finish" , "Long speech made by Saint Faith?" .)