Hidden facts about entering religious schools (7)
I believe the answer is:
secrets
'hidden facts' is the definition.
(I've seen this before)
'about entering religious schools' is the wordplay.
'about' becomes 're' (regarding).
'entering' means one lot of letters goes inside another.
'religious schools' becomes 'sects'.
're' placed into 'sects' is 'SECRETS'.
(Other definitions for secrets that I've seen before include "Confidential data" , "Confidential matters" , "Mysteries" , "Undivulged facts" , "Hidden facts" .)