Black arts of old Conservative party (6)
I believe the answer is:
occult
'black arts' is the definition.
(occult practices)
'old conservative party' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'conservative' becomes 'C' (UK political abbreviation).
'party' becomes 'cult' (cult can mean a sect or party).
'o'+'c'+'cult'='OCCULT'
'of' acts as a link.
(Other definitions for occult that I've seen before include "Involving the supernatural or magical" , "Mystical, involving the supernatural" , "Mystic" , "Magical; hide from view (astron.)" , "Hidden; mystical" .)