Magic Circle: sect must include foremost of conjurors (6)
I believe the answer is:
Here is my best explanation:
'magic' is the definition.
'circle sect must include foremost of conjurors' is the subsidiary indication.
'circle' becomes 'o'.
'sect' becomes 'cult' (I've seen this in other clues).
'must include' indicates putting letters inside.
'foremost of' indicates taking the first letters.
The initial letter of 'conjurors' is 'c'.
'cult' enclosing 'c' is 'ccult'.
(Other definitions for occult that I've seen before include "Secret - paranormal", "Magical; secret", "To do with the supernatural", "Esoteric, magical", "Involving the supernatural, mystical", "Mysterious", "Secret, esoteric", "Hidden - mysterious", "Secret - mysterious" and "Mysticism".)