Everything we can teach about religion, perhaps, is sound (3,5)
I believe the answer is:
all there
'is sound' is the definition.
(I've seen this before)
'everything we can teach about religion' is the wordplay.
I cannot really understand how this works, but
'everything' could be 'all' (all things) and 'all' is found in the answer.
'about' could be 're' (regarding) and 're' is found in the answer.
The remaining letters 'the' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'perhaps' acts as a link.
Can you help me to learn more?
(Other definitions for all there that I've seen before include "mentally alert" , "of normal intelligence" , "Nothing unaccounted for" , "Total number of people attending" , "Everybody attending the event" .)