Article on old Persian monarch's religious system (9)
I believe the answer is:
theosophy
'religious system' is the definition.
(I've seen this before)
'article on old persian monarch's' is the wordplay.
'article' becomes 'the' (the definite article in English).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'persian monarch' becomes 'sophy' (historical term for the ruler of Persia).
'the'+'o'+'sophy'='THEOSOPHY'
(Other definitions for theosophy that I've seen before include "religious system" , "Hope to shy (anag) - mystical belief" , "thought about religion" , "belief system" , "divine inspiration" .)