Once more, in religious groups, they're hush-hush (7)
I believe the answer is:
secrets
'hush-hush' is the definition.
The answer and definition are not the same part of speech.
'once more in religious groups' is the wordplay.
'once more' becomes 're' (prefix in words like 're-emerge' or 're-enter').
'in' indicates putting letters inside.
'religious groups' becomes 'sects' (sect is a type of religious group).
're' inserted into 'sects' is 'SECRETS'.
'they're' is the link.
Can you help me to learn more?
(Other definitions for secrets that I've seen before include "Confidential matters" , "Mysteries" , "They're not to be disclosed to others" , "Concealed facts" , "Confidential data" .)