Brief belief (3)
I believe the answer is:
ism
'brief belief' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ism that I've seen before include "Unspecified doctrine" , "Informal doctrine found in Bismarck" , "specific doctrine" , "Suffix denoting doctrine or practice" , "Any doctrine or theory" .)