Understood about source of confidence in belief that cannot be criticised (6,3)
I believe the answer is:
sacred cow
'that cannot be criticised' is the definition.
(something which cannot be criticised)
'understood about source of confidence in belief' is the wordplay.
I cannot quite see how this works, but
'about' could be 're' ('re' means regarding or about) and 're' is present in the answer.
'source' could be 'cow' (I have seen 'Source of milk ' mean 'cow' so perhaps 'source' could also mean 'cow') and 'cow' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for sacred cow that I've seen before include "One immune to criticism" , "A hallowed institution like Indian bovine" , "one not to be criticised" , "Facing this, carp not" , "counterpart of papal bull?" .)