Secret business taken up by religious sect (6)
I believe the answer is:
occult
'secret' is the definition.
(synonyms)
'business taken up by religious sect' is the wordplay.
'business' becomes 'co' (short for company).
'taken up' shows that the letters should be reversed in order.
'by' means one lot of letters go next to another.
'religious sect' becomes 'cult' (I've seen this in another clue).
'co' reversed gives 'oc'.
'oc'+'cult'='OCCULT'
(Other definitions for occult that I've seen before include "Mystical, involving the supernatural" , "Mysticism" , "Secret - supernatural" , "Magical; secret" , "Hidden - mysterious" .)