Particular system of belief (8)
I believe the answer is:
religion
'particular system of belief' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for religion that I've seen before include "Belief - way of life of monks and nuns" , "Institution for worship" , "Institution to express belief in a god" , "Organised system of faith" , "System of divine worship" .)