Cause of tears about religious belief (7)
I believe the answer is:
opinion
'belief' is the definition.
(I know that belief can be written as opinion)
'cause of tears about religious' is the wordplay.
'cause of tears' becomes 'onion' (I've seen this before).
'about' means one lot of letters goes inside another.
'religious' becomes 'pi' (british abbreviation for pious).
'onion' placed around 'pi' is 'OPINION'.
(Other definitions for opinion that I've seen before include "View held without positive proof" , "Conviction - point of view" , "Viewpoint" , "Estimation" , "Point of view or thought on something" .)