Closed book again held in religious group (6)
I believe the answer is:
secret
'closed book' is the definition.
'secret' can be an answer for 'closed' (thesaurus). I'm not sure about the 'book' bit.
'again held in religious group' is the wordplay.
'again' becomes 're' (prefix meaning 'again').
'held in' indicates putting letters inside.
'religious group' becomes 'sect' (eg a religious sect).
're' going within 'sect' is 'SECRET'.
Can you help me to learn more?
(Other definitions for secret that I've seen before include "Inner" , "Confidential or mysterious" , "Cloak and dagger" , "It's not to be told" , "avoiding exposure" .)