Turns out religious group rely on it behind closed doors (8)
I believe the answer is:
secretly
'behind closed doors' is the definition.
(I've seen this before)
'turns out religious group rely on' is the wordplay.
'turns out' becomes 't' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'religious group' becomes 'sec' (I've seen this in another clue).
'on' is an insertion indicator (as in clothing 'on' a person).
'sec'+'rely'='secrely'
't' put inside 'secrely' is 'SECRETLY'.
'it' acts as a link.
This may not be correct. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for secretly that I've seen before include "in disguise?" , "in camera" , "without telling anyone" , "behind closed doors" , "Covertly" .)