Hush-hush religious education in cult (6)
I believe the answer is:
secret
'hush-hush' is the definition.
(I know that hush-hush can be written as secret)
'religious education in cult' is the wordplay.
'religious education' becomes 'RE' (abbreviation).
'in' indicates putting letters inside.
'cult' becomes 'sect' (I've seen this before).
're' placed within 'sect' is 'SECRET'.
(Other definitions for secret that I've seen before include "Confidential (information)" , "'Hidden, covert (6)'" , "away from scrutiny" , "Concealed" , "Private, covert" .)