Secret old Conservative sect (6)
I believe the answer is:
occult
'secret' is the definition.
('occult' can be a synonym of 'secret')
'old conservative sect' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'conservative' becomes 'c' (abbreviation commonly used in UK politics).
'sect' becomes 'cult' (I've seen this in other clues).
'o'+'c'+'cult'='OCCULT'
(Other definitions for occult that I've seen before include "Hidden - paranormal" , "Mysticism" , "Secret - supernatural" , "Involving the supernatural or magical" , "Secret - mysterious" .)