Mysterious firm backed religious group (6)
I believe the answer is:
occult
'mysterious' is the definition.
(I know that mysterious can be written as occult)
'firm backed religious group' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'backed' is a reversal indicator.
'religious group' becomes 'cult' (I've seen this before).
'co' backwards is 'oc'.
'oc'+'cult'='OCCULT'
(Other definitions for occult that I've seen before include "Secret - supernatural" , "Esoteric, magical" , "Beyond ordinary understanding; difficult to see" , "Secret - paranormal" , "Mystical, involving the supernatural" .)