Article is on origin of mystical belief system (6)
I believe the answer is:
theism
'belief system' is the definition.
(I've seen this in another clue)
'article is on origin of mystical' is the wordplay.
'article' becomes 'the' (the definite article).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'origin of' suggests taking the first letters.
The first letter of 'mystical' is 'm'.
'the'+'is'+'m'='THEISM'
(Other definitions for theism that I've seen before include "Belief in existence of a God revealed to man" , "Belief in a deity" , "The doctrine or belief in the existence of a God or gods" , "of faith" , "Belief in a creator" .)